Allied Telesis Network Router X8100 User Manual

Technical Guide  
How To |Use Route Maps and Other Filters to Filter and Alter  
BGP and OSPF Routes  
Introduction  
ISPs transport large volumes of data. They often have to pay large amounts of money to  
transport their data through hired links, or through other providers' networks. Similarly, they  
can also charge money for transporting other ISPs' data through their network.  
Where significant amounts of money are involved, there are typically complex negotiations  
involved, and agreements made that are bound by all sorts of rules and restrictions and  
guarantees.  
Hence, ISPs need to be able to very precisely control which data gets sent and received on  
which links.This is achieved by having very precise control over the way the routing tables in  
their routers are built.  
To that end, the BGP implementation in AlliedWare Plus includes a set of facilities for filtering  
routes, and for altering the attributes that are associated with certain routes in BGP update  
messages. The main purpose of this document is to give an overview of these features, and  
touches on how to configure them. One of the central route manipulation facilities is the  
route map.  
Route maps can also be used for manipulating OSPF routes, so this document concludes by  
describing the use of route maps for OSPF.  
C613-16128-00 REV E  
alliedtelesis.com  
x
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Introduction  
Related How To Notes  
You can also configure route maps on routers and switches running AlliedWare. For details,  
see the AlliedWare Note How To Use Route Maps and Other Filters to Filter and Alter BGP and  
OSPF Routes. This Note is available from www.alliedtelesis.com/resources/literature/  
Which products and software version does it apply to?  
This configuration applies to AlliedWare Plus software version 5.2.2-0.4 and above, for the  
following Allied Telesis switches:  
SwitchBlade x8100 (CFC400, CFC960)  
SwitchBlade x908 and SBx908 Extended  
x600 and x610 Series Switches  
x900 Series switches  
Note: This document is a revision of an AlliedWare document, and also applies to the  
following AlliedWare products and software versions.  
AlliedWare Products: AR440S, AR441S, AR450S, AR725, AR745, AR750S, Rapier i series,  
AT-8800 series, AT-9800 series, AT-8948, AT-9924T, AT-9924SP, AT-9924T/4SP, AT-9924Ts,  
x900 series.  
AlliedWare Software version: 2.7.4 and above.  
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BGP: Concepts and Terminology  
BGP: Concepts and Terminology  
Before moving on to look at the filtering processes, it is important to first have some  
understanding of certain aspects of how BGP works. The following sections describe:  
BGP peers  
BGP updates  
Update attributes  
BGP peers  
Definition Within the BGP protocol, the exchange of routing information is carried out between pairs  
of routers. Two routers create a TCP connection with each other, and exchange routing  
information as specific data packets within that TCP session. The routers at the ends of the  
TCP connection are referred to as BGP peers. Any given router can form peering  
relationships with multiple routers.  
Usually a BGP router with an ISP will form peer relationships with BGP routers at other ISPs  
or clients with which it has entered into data transporting agreements.  
The process of BGP filtering usually comes down to a matter of specifying the routes that will  
be sent to, or received from, each of a router's peers.  
BGP updates  
Definition Once a router has established a BGP connection with a peer, it will start to exchange routing  
information with that peer. A BGP update message is the packet that is used to transfer the  
routing information.  
The routing information contained within an update message consists of:  
a set of attribute values (see the next section for a description of the possible attributes)  
and  
a list of one or more prefixes. A prefix is the network portion of an IP address, in dotted  
decimal notation, optionally followed by a "/" character and a decimal number from 0 to  
32. Each prefix contained within an update message represents a network that can be  
reached through the IP address given in the NextHop attribute contained in the same  
update message.  
Note: There is only one NextHop attribute in an update message, so all the routes in the  
update message have the same next hop.  
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BGP: Concepts and Terminology  
Update attributes  
As mentioned above, each BGP update message contains a set of attributes. These attributes  
describe some of the properties of the routes, and can be used in making decisions about  
which routes to accept and which to reject. Some of the attributes are:  
Origin  
How a prefix came to be routed by BGP at the origin Autonomous System (AS). Prefixes are  
learned from various sources such as directly connected interfaces, manually configured static  
routes, or dynamic internal or external routing protocols, and then put into BGP.  
AS-path  
The list of Autonomous Systems (ASs) through which the announcement for the prefix has  
passed. As prefixes pass between Autonomous Systems each one adds its Autonomous  
System Number (ASN).  
Next-hop  
The address of the next node that the router should send packets destined for the specified  
prefixes to, in order to get the packets closer to the destination.  
Multi-Exit-Discriminator (MED)  
A metric expressing the optimal path to reach a particular prefix in or behind a particular AS.  
Local-preference  
A metric used in IBGP so each host knows which path inside the AS it should use to reach  
the advertised prefix. EBGP peers do not send this value, and ignore it on receipt.  
Atomic-aggregate  
A non-transitive attribute that allows BGP peers to inform each other about decisions they  
have made regarding overlapping routes. Non-transitive means that if the attribute is  
received by a device that does not recognise the attribute, it is dropped and not passed on  
to the next router.  
Aggregator  
Can be attached to an aggregated prefix to specify the AS and router that performed the  
aggregation.  
Community  
Indicates where a prefix is relevant to—for example, if it is relevant to the whole Internet, or  
just within an AS.  
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BGP: Overview of the Available Filter Types  
BGP: Overview of the Available Filter Types  
The following sections describe the various types of filters that can be applied to BGP  
updates and the hierarchy of the filters.  
Filter types  
There are a number of filter types that can be applied to the BGP updates being exchanged  
between BGP peers:  
Distribute filters  
Distribute list can filter the routing information between the routing protocol (RIP, RIPng,  
OSPF, OSPFv3 or BGP) and its IP route table (routing information base - RIB). It has to make  
the use of ACL, prefix list, or route map depending on which routing protocol it is configured  
for. For BGP, it can only use ACL to match prefix (ip address).  
These use ACLs and look at the individual prefixes within an update message. If a prefix  
within the update message matches the filter criteria then that individual prefix is filtered out  
or accepted depending on what action the filter entry has been configured to carry out.  
Note that you cannot combine distribute filters and prefix filters.  
AS Path filters  
These look at the AS-Path attribute in update messages. If the AS-Path attribute in the  
update matches the filter criteria then the whole update message is filtered out or accepted,  
depending on what action the filter entry has been configured to carry out.  
Prefix filters  
These use prefix lists and look at the individual prefixes within an update message. If a prefix  
within the update message matches the filter criteria then that individual prefix is filtered out  
or accepted depending on what action the filter entry has been configured to carry out.  
Note that you cannot combine distribute filters and prefix filters.  
Route maps  
These are a structured combination of match criteria and actions. They can be used to filter  
out routes and also to alter the attributes in update messages.  
Access Control List (ACL) filters  
ACL is mainly use to limit the network traffic. It can be used by distribute list and route map  
to filter the routing updates. ACLs (Access Control Lists) can be used to do a number of  
different things on your network, from controlling what type of traffic passes through your  
devices to deciding on what routing protocols you wish to be advertised out of your routers.  
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BGP: Overview of the Available Filter Types  
Difference and Relationship in BGP  
KEY WORD IN COMMAND DEFINABLE FILTER  
WAY TO APPLY FILTERING  
ACL  
access-list  
distribute-list  
prefix-list  
as-path  
Yes  
Distribute list  
Prefix list  
AS path list  
Filter list  
Yes  
Yes  
Yes  
Yes  
filter-list  
Yes  
Yes  
Route map  
route-map  
Yes  
DEFINABLE  
FILTER  
WAY TO APPLY FILTERING  
DISTRIBUTE LIST  
PREFIX LIST  
FILTER LIST  
ROUTE MAP  
YES  
ACL  
YES  
Prefix list  
AS path list  
Route map  
YES  
YES  
YES  
YES  
YES  
Examples for filtering BGP Update messages  
All above filters can be used in incoming or outgoing directions of the update messages. For  
ACL, distribute list, AS path list, and prefix list, the order of application is not important. If an  
update is denied by any given filter, it is discarded immediately, and is not run through any of  
the other filters. If an update is permitted by one filter, it is passed through to the next filter  
to be considered. At the end, you end up with the set of updates that all the filters agree  
should not be discarded. However, route maps are applied last, after the other types of filter.  
This is because route maps can modify updates, not just accept or discard them.  
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Hierarchy of the Different Filters  
Hierarchy of the Different Filters  
For distribute filters (ACLs), path filters, and prefix filters, the order of application is not  
important. If an update is denied by any given filter, it is discarded immediately, and is not run  
through any of the other filters. If an update is permitted by one filter, it is passed through to  
the next filter to be considered. At the end, you end up with the set of updates that all the  
filters agree should not be discarded.  
However, route maps are applied last, after the other types of filter. This is because route  
maps can modify updates, not just accept or discard them.  
BGP: Example  
This How To Note illustrates each type of filter with an example from a very simple BGP  
network. This section describes the basic network configuration. The following sections add  
filtering options to it:  
The network consists of two BGP peers in different Autonomous Systems (AS):  
port1.0.2, vlan 64:  
64.64.64.64/4  
Switch running  
AlliedWare Plus  
port1.0.1, vlan1:  
45.45.45.45/24  
ASN 34567  
port 50, vlan 1:  
45.45.45.46/8  
Switch running  
AlliedWare  
port 52, vlan 2:  
52.52.52.5/8  
ASN 34568  
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Hierarchy of the Different Filters  
Basic configuration  
This configuration gets the neighbor relationship established and some routes exchanged.  
AlliedWare Plus Create the second VLAN and associate port1.0.2 with it; assign IP addresses; and configure  
switch  
BGP.  
vlan database  
vlan 64 name v64  
interface port1.0.2  
switchport access vlan 64  
interface vlan1  
ip address 45.45.45.45/24  
interface vlan64  
ip address 64.64.64.64/4  
router bgp 34567  
redistribute connected  
neighbor 45.45.45.46 remote-as 34568  
AlliedWare Create the second VLAN and associate port52 with it; assign IP addresses; and configure  
switch  
BGP.  
create vlan="v2" vid=2  
add vlan="2" port=52  
enable ip  
set ip autonomous=34568  
add ip int=vlan1 ip=45.45.45.46  
add ip int=vlan2 ip=52.52.52.5  
set bgp ro=45.45.45.46  
add bgp pe=45.45.45.45 rem=34567  
ena bgp pe=45.45.45.45  
add bgp imp=interface  
Note: The prefix lengths on the UIP addresses on VLAN1 of the two switches are different.  
The VLAN1 address on the AlliedWarePlus switch has prefix length 24, and that on  
the AlliedWare switch has prefix length 8. Although this is a non-standard, and not  
recommended, configuration, it has been done deliberately for the purposes of the  
illustrative examples in this How To Note. By having different prefix lengths on those  
addresses, it is clear in the route tables of the two switches which of the 45.x.x.x  
routes is a connected route, and which is a BGP-learnt route.  
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Hierarchy of the Different Filters  
Confirming the neighbor relationship  
Check that each switch sees the interface route advertised from the other switch. On both  
the AlliedWare Plus and AlliedWare switches, use the command show ip route.  
AlliedWare Plus  
switch  
awplus#show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
B
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:18:10  
45.45.45.0/24 is directly connected, vlan1  
52.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:18:10  
64.0.0.0/4 is directly connected, vlan64  
AlliedWare  
switch  
Manager BGP peer> show ip route  
IP Routes  
---------------------------------------------------------------------  
Destination Mask  
Type  
NextHop  
Policy Protocol  
Flags Interface  
Metrics  
Age  
Pref  
Tag  
---------------------------------------------------------------------  
45.0.0.0  
255.0.0.0  
direct 0  
0.0.0.0  
interface -  
----- vlan1  
1
2128  
0
45.45.45.0  
993  
255.255.255.0 45.45.45.45  
----- vlan1  
remote 0  
255.0.0.0  
direct 0  
240.0.0.0  
bgp  
0.0.0.0  
interface -  
45.45.45.45  
-
2
----- vlan2  
1
170  
1642  
0
52.0.0.0  
64.0.0.0  
993  
S---- vlan1  
remote 0  
bgp  
-
2
170  
--------------------------------------------------------------------  
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BGP: Configuring Distribute Filters  
BGP: Configuring Distribute Filters  
Distribute filters use ACLs (Access Control Lists) to filter particular routes on the basis of  
their prefixes. Distribute filters and prefix filters both filter individual routes out of BGP  
update packets. They are mutually exclusive.  
About ACLs  
From the point of view of route filtering, an ACL is one or more simple unnumbered filter  
entries, each with a prefix and an action of deny or permit.  
You can use any of the following syntax options to create the ACL entries. The main  
difference is in how you label the ACL—whether you use a name or a number.  
access-list standard <name> {deny|permit} <ipadd/prefixlength>  
exact-match  
access-list <1-99> {deny|permit} <ipadd> <reverse-mask>  
access-list <1300-1999> {deny|permit} <ipadd> <reverse-mask>  
Entries are unnumbered, so each new entry gets added to the end of the ACL.  
Named ACLs Using a standard named ACL lets you specify whether the prefix needs to be an exact match  
or not. If you specify exact-match, then routes only match the ACL if they have the specified  
prefix length. Otherwise, routes match the ACL if they have a prefix length equal to or longer  
than the specified prefix length. For example, if you specify 10.0.0.0/8, then:  
without exact-match, the ACL matches all of 10.0.0.0/8–10.0.0.0/32  
with exact-match, the ACL only matches 10.0.0.0/8  
Numbered For numbered ACLs, the mask is a reverse (or wildcard) mask. This is the opposite of a  
ACLs  
standard mask in dotted decimal notation. However—in line with industry standards—the  
mask value has no effect. The ACL always applies to all prefix lengths.  
Extended ACLs You can also use an extended ACL (number range 100-199, or 2000-2699, or by using the  
extended <name> parameter) but there is no advantage to doing so. Extended ACLs  
include two prefixes (source and destination), and using two prefixes is meaningless when  
filtering routes.  
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BGP: Configuring Distribute Filters  
Using ACLs as filters  
When you have created an ACL, you can use it to filter incoming or outgoing update  
messages for a particular BGP peer, by using the following commands in BGP router mode  
for the AS.  
Filter incoming updates (received from a particular neighbor):  
awplus(config-router)#  
neighbor <neighbor> distribute-list  
<acl-id> in  
Filter outgoing updates (destined for a particular neighbor):  
awplus(config-router)#  
neighbor <neighbor> distribute-list  
<acl-id> out  
The switch will then compare the prefixes in update packets with each entry in the ACL,  
looking for matches.  
If a matching entry has the parameter permit, then there will be effectively no action. If a  
matching entry has the parameter deny, then the specified prefix will be removed from the  
update packet.  
Once the update packet has been compared against every entry in the ACL, it will be sent to  
the neighbor (out filters) or processed (in filters), minus any prefixes that have been removed  
by the filter.  
Example: Distribute filters  
Filter out one particular route from a neighbor  
This example expands on the basic configuration in "BGP: Example" on page 8.  
It creates an ACL on the AlliedWare Plus switch that explicitly denies one of the routes that  
is advertised from the AW neighbor, and explicitly accepts all other routes.  
1. Create a named ACL to deny the route 52.0.0.0/8 and accept all others. You need  
to include a permit any entry because ACLs end in an implicit deny any entry.  
awplus(config)#  
awplus(config)#  
access-list standard list1 deny 52.0.0.0/8 exact  
access-list standard list1 permit any  
2. Set that ACL as the filter for the BGP neighbor 45.45.45.46.  
awplus(config)#  
router bgp 34567  
awplus(config-router)#  
neighbor 45.45.45.46 distribute-list list1 in  
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BGP: Configuring Distribute Filters  
3. Renew the route exchange by shutting down the neighbor, then bring it up again.  
awplus(config-router)#  
awplus(config-router)#  
neighbor 45.45.45.46 shutdown  
neighbor 45.45.45.46 no shutdown  
4. Check that the IP route table no longer includes 52.0.0.0/8.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:00:14  
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
Filter out a range of prefix lengths  
This example demonstrates the effect of the exact parameter in the ACL by discarding all  
routes to 52.0.0.0 with prefix lengths of 4 or greater.  
1. Remove the existing ACL..  
awplus(config)#  
no access-list standard list1  
2. Shut down the neighbor, and then bring it up again.  
awplus(config-router)#  
awplus(config-router)#  
neighbor 45.45.45.46 shutdown  
neighbor 45.45.45.46 no shutdown  
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BGP: Configuring Distribute Filters  
3. Check that the IP route table now includes all the routes.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
B
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:01:57  
45.45.45.0/24 is directly connected, vlan1  
52.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:01:57  
64.0.0.0/4 is directly connected, vlan64  
4. Add the following ACL entries. When exact is not specified, the ACL entry  
matches all masks greater than or equal to the specified mask, so the first entry  
below would block routes like 52.0.0.0/8.  
awplus(config)#  
awplus(config)#  
access-list standard list2 deny 52.0.0.0/4  
access-list standard list2 permit any  
5.  
Set that ACL as the filter for the BGP neighbor 45.45.45.46:  
awplus(config)#  
awplus(config)#  
router bgp 34567  
neighbor 45.45.45.46 distribute-list list2 in  
6. Shut down the neighbor, and then bring it up again.  
awplus(config-router)#neighbor 45.45.45.46 shutdown  
awplus(config-router)#neighbor 45.45.45.46 no shutdown  
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BGP: Configuring Distribute Filters  
7. Check that the IP route table no longer includes 52.0.0.0/8.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:00:08  
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
Use a numbered ACL instead of a named ACL  
This example demonstrates a numbered ACL by discarding all routes to 52.0.0.0.  
1. Create a numbered ACL.  
awplus(config)#  
awplus(config)#  
access-list 1301 deny 52.0.0.0 0.0.0.255  
access-list 1301 permit any  
In line with industry standards, the wildcard mask is required but its value has no effect. The  
ACL always applies to all prefix lengths.  
2. Set that ACL as the filter for the BGP neighbor 45.45.45.46.  
awplus(config)#  
router bgp 34567  
awplus(config-router)#  
neighbor 45.45.45.46 distribute-list 1301 in  
3. Shut down the neighbor, and then bring it up again.  
awplus(config-router)#  
awplus(config-router)#  
neighbor 45.45.45.46 shutdown  
neighbor 45.45.45.46 no shutdown  
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BGP: Configuring Distribute Filters  
4. Check that the IP route table no longer includes 52.0.0.0/8.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:05:30  
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
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BGP: Configuring AS Path Filters  
BGP: Configuring AS Path Filters  
To configure path filters we need to first understand something about AS path lists and how  
to use them.  
AS path lists  
Path filters use a construct known as an AS path list. An AS path list has a name and consists  
of one or more (unnumbered) entries. Each entry specifies:  
which AS paths to consider.  
whether the AS paths in question should be included or excluded from the list.  
The set of paths to consider is specified using regular expressions. The AlliedWare Plus OS  
supports the full set of syntax elements for simple regular expressions:  
SYMBOL  
CHARACTER  
USED TO...  
^
Caret  
Match the beginning of the input string. When  
used at the beginning of a string of characters, it  
negates a pattern match.  
$
.
Dollar sign  
Period  
Match the end of the input string.  
Match a single character (white spaces included).  
Match zero or more sequences of a pattern.  
Match one or more sequences of a pattern.  
Match zero or one occurrences of a pattern.  
*
+
?
Asterix  
Plus sign  
Question mark  
Underscore  
_
Match spaces, commas, braces, parentheses, or the  
beginning and end of an input string.  
[]  
-
Brackets  
Hyphen  
Specify a range of single-characters.  
Separate the end points of a range.  
For example, you can specify things like:  
any path that contains a certain set of AS numbers: e.g. 23334 45634 8988  
any path that starts with a particular AS number: e.g. ^23334  
any path that ends with a particular AS number: e.g. 8988$  
a specific path: e.g. ^23334 45634 8988$  
an empty path: ^$  
Syntax To create an entry in an AS path list, use the command:  
awplus(config)#  
ip as-path access-list <list-name> {deny|  
permit} <regular-expression>  
There is an implicit deny all entry at the end of each path list. Any AS path that does not  
match any earlier entry is excluded from the list.  
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BGP: Configuring AS Path Filters  
Using AS path lists as path filters  
When an AS path list has been created, it can be applied to filter incoming or outgoing  
update messages for a particular BGP peer, by using the following commands in BGP router  
mode for the AS.  
Filter incoming updates (received from a particular neighbor):  
awplus(config-router)#  
neighbor <neighbor> filter-list <list-name> in  
Filter outgoing updates (destined for a particular neighbor):  
awplus(config-router)#  
neighbor <neighbor> filter-list  
<list-name> out  
The router will then compare the AS path attribute in BGP update packets with each entry  
in the AS path list until a match is found. If the AS path list entry that matches has the  
parameter permit, then the update packet will be allowed through by the filter. If the  
matching entry has the parameter deny, then the update packet will be blocked by the filter.  
Note: All update packets whose AS paths do not explicitly match an entry in the AS path  
list will be dropped, because the list ends in an implicit deny all entry.  
Example: AS path filters  
Discard or allow routes from a neighbor  
This example expands on the basic configuration in "BGP: Example" on page 8.  
First, it creates an AS path filter on the AlliedWare Plus switch that explicitly includes only AS  
23456. AS path filters end in an implicit deny clause, so this filter implicitly excludes  
AS 34568. After demonstrating that AS 34568 is excluded, the example then adds an entry  
to the filter to explicitly allow AS 34568.  
1. If you previously configured a distribute filter, as shown in "Example: Distribute  
filters" on page 12, remove it from the neighbor definition.  
awplus(config)#  
router bgp 34567  
awplus(configrouter)#no neighbor 45.45.45.46 distribute-list  
1301 in  
Implicitly  
exclude a  
neighbor  
2. Create an AS path access list that includes AS 23456.  
awplus(config)#  
ip as-path access-list list1 permit 23456  
3. Set that access list as the in-filter for the BGP neighbor 45.45.45.46.  
awplus(config)#  
router bgp 34567  
awplus(config-router)#  
neighbor 45.45.45.46 filter-list list1 in  
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BGP: Configuring AS Path Filters  
4. Shut down the neighbor, and then bring it up again.  
awplus(config-router)#  
awplus(config-router)#  
neighbor 45.45.45.46 shutdown  
neighbor 45.45.45.46 no shutdown  
5. Check that the IP route table does not have the BGP routes from the AlliedWare  
neighbor in AS 34568 any more.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
C
C
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
Explicitly include  
a neighbor  
6. Add an entry to the an AS path access list that explicitly includes AS 34568.  
awplus(config)#  
ip as-path access-list list1 permit 34568  
7. Shut down the neighbor, and then bring it up again.  
awplus(config-router)#  
awplus(config-router)#  
neighbor 45.45.45.46 shutdown  
neighbor 45.45.45.46 no shutdown  
8. Check that the IP route table has the BGP routes from the AlliedWare neighbor in  
AS 34568 again.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
B
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:00:03  
45.45.45.0/24 is directly connected, vlan1  
52.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:00:03  
64.0.0.0/4 is directly connected, vlan64  
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BGP: Configuring AS Path Filters  
9. Check that the AS path list shows the two filter entries:  
awplus(config-router)#  
do show ip as-path-access-list  
AS path access list list1  
deny 23456  
permit 34568  
Another example  
An outgoing  
filter that uses  
an AS-path list  
1. Create an AS-PATH list that denies empty AS Paths, but allows AS Paths that  
contain the AS number 34567.  
ip as-path access-list example deny ^$  
ip as-path access-list example permit 34567  
2. Apply this as the out route map for neighbor 45.45.45.46.  
neighbor 45.45.45.46 filter-list example out  
3. Shut down the neighbor, and then bring it up again.  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
4. There are no routes advertised to the other side, so that says that the AS Path in  
the update packets is not 34567 at the point where the updates are passed to the  
out-going filter.  
5. Now remove the deny entry for empty AS-paths, and add an allow entry for empty  
AS_paths.  
(config)#no ip as-path access-list example deny ^$  
(config)#ip as-path access-list example permit ^$  
The as-path list becomes:  
ip as-path access-list example permit 34567  
ip as-path access-list example permit ^$  
6. Shut down the neighbor, and then bring it up again.  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
Now the routes are received by the neighbor. That does demonstrate that the AS-paths in  
the updates are empty at the moment they are examined by the filter. But, in the neighbor's  
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BGP: Configuring AS Path Filters  
BGP table, the AS Path for the learnt routes is shown as "SEQ 34567". This entry is inserted  
into the AS-Path of the updates after they have passed through the filter.  
BGP route table  
Flags: >=Best route for the given prefix, *=Unreachable next hop,  
W=Withdrawn  
m=Community, a=Aggregate route, s=Aggregate Suppressed, D=Damped  
Learned from: L=Local, e=eBGP Peer, i=iBGP Peer, c=Confederate  
Peer  
---------------------------------------------------------------------  
Fl Prefix  
Path  
Next hop  
Origin  
MED  
Local pref  
Originator  
Cluster List  
---------------------------------------------------------------------  
> 45.0.0.0/8  
EMPTY  
L -  
> 45.45.45.0/24  
SEQ 34567;  
e -  
> 52.0.0.0/8  
EMPTY  
0.0.0.0  
IGP  
-
100  
100  
100  
100  
-
45.45.45.45  
INCOMPLETE -  
-
0.0.0.0  
IGP  
-
L -  
-
> 64.0.0.0/4  
SEQ 34567;  
e -  
45.45.45.45  
INCOMPLETE -  
-
---------------------------------------------------------------------  
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BGP: Configuring Prefix Filters  
BGP: Configuring Prefix Filters  
Prefix filters use prefix lists to filter particular routes on the basis of their prefixes. Prefix filters  
and distribute filters both filter individual routes out of BGP update packets. They are  
mutually exclusive.  
About prefix lists  
A prefix list is a list of prefix entries. Each entry specifies a particular prefix, a mask length or  
range of mask lengths, and whether or not those prefixes are deemed to explicitly match or  
explicitly not match the prefix list.  
A prefix list entry is created with the command:  
awplus(config)#  
ip prefix-list <list-name> [seq <number>]   
{deny|permit} {any|<ipadd>/<prefix-length>}   
[ge <minimum-length>] [le <maximum-length>]  
You can choose to give an entry a sequence number by using the optional seq parameter. If  
you do not, the switch assigns sequence numbers in steps of 5 (number 5, 10, 15 etc.) and  
puts the new entry at the end of the list of entries.  
To see entries and their numbers, use the command:  
awplus#  
show ip prefix-list  
Mask length  
You can specify a single prefix mask length, or you can use the ge and le parameters to  
specify a range of mask lengths for the entry to match.  
If you set the mask length to:  
a single mask length (by specifying neither the ge nor the le parameter), then a route  
matches against this entry if its prefix mask length is exactly that length.  
a range of mask lengths, then a route matches against this entry if its prefix mask length is  
greater than or equal to ge and less than or equal to le.  
For example, to deny the IP addresses between 10.0.0.0/14 (mask of 255.252.0.0) and  
10.0.0.0/22 (mask of 255.255.252.0) within the 10.0.0.0/8 (mask of 255.0.0.0) addressing  
range, use the command:  
awplus(config)#  
ip prefix-list mylist deny 10.0.0.0/8 le 22 ge 14  
The mask length (8 in this example) must be less than the value specified for the ge  
parameter. This mask defines the range of subnets that the matching subnets must fall within.  
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BGP: Configuring Prefix Filters  
Using prefix lists as prefix filters  
When you have created a prefix list, you can use it to filter incoming or outgoing update  
messages for a particular BGP peer, by using the following commands in BGP router mode  
for the AS.  
Filter incoming updates (received from a particular neighbor):  
awplus(config-router)#  
neighbor <neighbor> prefix-list <list-name> in  
Filter outgoing updates (destined for a particular neighbor):  
awplus(config-router)#  
neighbor <neighbor> prefix-list <list-name>  
out  
The router will then compare the prefixes in update packets with each entry in the prefix list,  
looking for matches.  
If a matching entry has the parameter permit, then there will be effectively no action. If a  
matching entry has the parameter deny, then the specified prefix will be removed from the  
update packet.  
Once the update packet has been compared against every entry in the prefix list, it will be  
sent to the neighbor (out filters) or processed (in filters), minus any prefixes that have been  
removed by the filter.  
Example: Prefix filters  
Filter out one particular route from a neighbor  
This example expands on the basic configuration in "BGP: Example" on page 8.  
It creates a prefix list on the AlliedWare Plus switch that explicitly permits one of the routes  
that is advertised from the AW neighbor, and therefore implicitly denies the other route.  
1. If you previously configured an AS path filter, as shown in "Example: AS path  
filters" on page 18, remove it from the neighbor definition.  
awplus(config)#  
router bgp 34567  
awplus(config-router)#  
no neighbor 45.45.45.46 filter-list list1 in  
2. Create an IP prefix list to include the route 45.0.0.0/8. Prefix lists end in an implicit  
exclude clause, so this will exclude all other routes.  
awplus(config)#  
ip prefix-list list1 permit 45.0.0.0/8  
3. Set that prefix list as the prefix list filter for the BGP neighbor 45.45.45.46.  
awplus(config)#  
router bgp 34567  
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BGP: Configuring Prefix Filters  
awplus(config-router)#  
neighbor 45.45.45.46 prefix-list list1 in  
4. Shut down the neighbor, and then bring it up again.  
awplus(config-router)#  
awplus(config-router)#  
neighbor 45.45.45.46 shutdown  
neighbor 45.45.45.46 no shutdown  
5. Check that the IP route table now contains one of the routes learnt from that  
neighbor, but not the other.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:00:06  
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
Filter out a range of different prefix lengths  
This example filters out routes to 45.0.0.0 with prefix lengths in the range 5-8, accepts routes  
to 52.0.0.0/8, and implicitly filters out all other BGP routes.  
1. Remove the existing prefix list entry.  
awplus(config)#  
no ip prefix-list list1 permit 45.0.0.0/8  
2. Add the following prefix list entries.  
awplus(config)#  
awplus(config)#  
ip prefix-list list1 deny 45.0.0.0/4 ge 5 le 8  
ip prefix-list list1 permit 52.0.0.0/8  
3. Set that prefix list as the prefix list filter for neighbor 45.45.45.46  
awplus(config)#  
awplus(config)#  
router bgp 34567  
neighbor 45.45.45.46 prefix-list list2 in  
4. Shut down the neighbor, and then bring it up again.  
awplus(config-router)#  
awplus(config-router)#  
neighbor 45.45.45.46 shutdown  
neighbor 45.45.45.46 no shutdown  
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BGP: Configuring Prefix Filters  
5. Check that the IP route table now contains only the other route from the  
neighbor.  
awplus(config-router)#  
do show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
C
B
C
45.45.45.0/24 is directly connected, vlan1  
52.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:26:45  
64.0.0.0/4 is directly connected, vlan64  
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BGP: Configuring Route Maps  
BGP: Configuring Route Maps  
Route maps are very powerful and flexible entities. Therefore, the configuring of route maps  
must, by necessity, be relatively complex. The purpose of this section of the document is to  
understand route maps piece by piece and thereby build up a full understanding of how all  
the parts fit together.  
Structure of a route map  
There are various levels of structure within a route map:  
A route map is an entity with a name  
Each route map consists of multiple entries, identified by sequence numbers  
Each entry can consist of multiple clauses  
In effect, an entry defines an individual filter. It can have a match clause that defines what it  
will match on, and it can have multiple set clauses that can specify actions to be taken. An  
update packet is matched against each entry in turn. Once an entry is found that matches the  
packet, the action(s) associated with that entry is (are) performed, and no further entries are  
considered.  
For example, if you create an entry that will permit an update packet, followed by an entry  
that would deny that packet, the packet is permitted. As another example, if you create two  
conflicting set clauses, in different entries, the first change is applied, not the second.  
A route-map consists of an ordered set of entries. Each entry is configured by commands of  
the form:  
route-map <map-name> permit|deny <sequence number>  
<match clause>  
<set clause(s)>  
There is no need for a command that creates a route-map prior to entries being created in  
the route-map. Simply, the first time an entry is created for route-map <name> then that  
route-map comes into existence.  
The sequence numbers determine the order in which the entries are applied to update  
packets. A route-map entry is not required to include a match clause. An entry with no  
match clause will match every date packet.  
There is an implicit match all filter at the end of the route map. The action on that implicit  
entry is deny. By default, any update packet that does not explicitly match any particular  
entry in the route map will be dropped.  
You can change this by ending the route map with a “permit all” clause, such as the following:  
awplus(config)#  
route-map <map-name> permit 65535  
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BGP: Configuring Route Maps  
Clauses  
There are two types of clauses that can be present in a route map entry:  
match clauses, which specify attributes or prefixes to match on  
set clauses, which specify the changes to be made to attribute values  
A given route map entry can never have more than one match clause, but it can have  
multiple set clauses.  
Configuring a match clause  
When you configure a match clause, you can match on one of the attributes listed in the  
following sections.  
An AS path list  
For information about creating an AS path list, see "AS path lists" on page 17.  
Once you have made the path list, you can apply it to the match clause of a route map entry  
by using the commands:  
awplus(config)#  
route-map <map-name> {deny|permit} <seq>  
match as-path <list-name>  
awplus(config-route-map)#  
This clause will cause the router to go through the entries in the AS path list. If one of the  
entries in the AS path list matches the AS path in the update packet, and the action on that  
AS path list entry is permit, then the packet is deemed to have matched this route map  
match clause. The fact that there are two levels of matching going on, and effectively two  
different uses of the permit/deny parameters in the one clause, can be a bit confusing.  
When working out the correct way to configure a clause that matches on AS path list, it is  
important to first think about how you are configuring the match criteria within the AS path  
list, and then separately think about the actions of the route map entry.  
In particular, it is important to note that in this context, the parameters in the AS path list do  
not indicate whether the matching update packet (remember that the AS Path attribute is  
only set once in an update packet) is being allowed or dropped; "permit" and "deny" simply  
indicate whether the update packet is deemed to match or not match the AS path list. This is  
different to the case where the AS path list itself is being used as a filter.  
Just as an example, consider these configurations:  
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BGP: Configuring Route Maps  
Case 1:  
awplus(config)#  
awplus(config)#  
awplus(config)#  
ip as-path access-list example deny ^$  
ip as-path access-list example permit 15557  
router bgp 100  
awplus(config-router)#  
neighbor 192.168.200.201 filter-list example  
out  
Case 2:  
awplus(config)#  
ip as-path access-list example permit ^$  
ip as-path access-list example deny 15557  
route-map rmapexample deny 1  
match as-path example  
awplus(config)#  
awplus(config)#  
awplus(config-route-map)#  
awplus(config-route-map)#  
awplus(config-route-map)#  
awplus(config-router)#  
route-map rmapexample permit 65535  
router bgp 100  
neighbor 192.168.200.201 route-map  
rmapexample out  
Both of these configurations would cause outgoing update packets with empty AS paths to  
be dropped, and update packets with an AS path containing 15557 to be allowed.  
But, to achieve that in the second case, the AS path list has to be configured to permit empty  
paths. This way, the empty path will match the AS path list, and be included into the route  
map's action of dropping packets that match the AS path list.  
In the second case, the AS path list specifically excludes packets whose AS path contains  
15557. Therefore, these packets are not dropped by the first route map entry, and are  
permitted by the last (permit all) route map entry.  
A community list  
Creating A community list is a set of entries that specify which community attribute values are  
included in or excluded from the list.  
There are two types of community list: standard and expanded.  
Standard lists are a just a list of one or more communities. They can be identified by a name,  
a number, or the word standard, and are created by using any of the following commands:  
awplus(config)#  
awplus(config)#  
awplus(config)#  
ip community-list <1-99> {deny|permit} {aa:xx|  
internet|local-as|no-advertise|no-export}  
ip community-list <list-name> {deny|permit} {aa:xx|  
internet|local-as|no-advertise|no-export}  
ip community-list standard <list-name>   
{deny|permit} {aa:xx|internet|local-as|  
no-advertise|no-export}  
If you have more than one community in an entry, separate them with spaces.  
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BGP: Configuring Route Maps  
Expanded lists use regular expressions to specify the communities. They can be identified by  
a number, or by the word expanded, and are created by using any of the following  
commands:  
awplus(config)#  
ip community-list <100-199> {deny|permit}  
<reg-exp>  
awplus(config)#  
ip community-list expanded <list-name>   
{deny|permit} <reg-exp>  
You can have multiple entries in a community list. Entries are unnumbered, so each new  
entry gets added at the end of the list.  
There is an implicit exclude all else entry at the end of the community list.  
Applying Once you have created a community list, use it in a route map entry by using the command:  
awplus(config-route-map)#  
match community <list> [exact-match]  
The optional exact-match parameter specifies that the communities contained in the  
attribute section of the update message must exactly match the specified community list. If  
you do not specify exact-match, then the set of communities in the attribute list of the  
update message must include all the communities in the specified community list, but can also  
include other communities.  
One or more prefixes, by using a prefix list  
For information about creating a prefix list, see "About prefix lists" on page 22.  
Once you have made the prefix list, apply it to the match clause of a route map entry by  
using the command:  
awplus(config-route-map)#  
match ip address prefix-list <list-name>  
A prefix list can match a subset of prefixes in an update message. You can use this to change  
the attributes of some of the prefixes in an outgoing update, without having to change the  
attributes of all the prefixes. However, an update message contains just one set of attributes,  
which must apply to all the prefixes in the update.  
If a route map entry matches on some of the prefixes in a particular outgoing update  
message, and has an associated set clause that specifies a change to the attributes for those  
prefixes, then the switch splits the update into two update packets:  
one that contains the original attribute values and the prefixes that were not included by  
the route map entry, and  
one that contains the new attribute values and the prefixes that were included by the  
route map entry.  
One or more prefixes, by using an ACL  
An ACL is an alternative to a prefix list for matching a prefix in an update message.  
For information about creating an ACL, see "About ACLs" on page 11.  
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BGP: Configuring Route Maps  
Once you have made the ACL, apply it to the match clause of a route map entry by using the  
command:  
awplus(config-route-map)#  
match ip address <acl-number-or-name>  
A next hop address  
You can use either a prefix list or an ACL to specify a next hop address. Once you have made  
the prefix list or ACL, apply it to the match clause of a route map entry by using one of the  
commands:  
awplus(config-route-map)#  
awplus(config-route-map)#  
match ip next-hop prefix-list <list-name>  
match ip next-hop <acl-number-or-name>  
An update message matches this route map entry if the next-hop attribute in the update  
equals an IP address permitted in the prefix list or ACL.  
An origin  
To match an origin, use the command:  
awplus(config-route-map)#  
match origin {egp|igp|incomplete}  
An update message matches this route map entry if the origin attribute in the update equals  
the origin specified in the entry.  
A metric (the MED attribute)  
To match the MED value, use the command:  
awplus(config-route-map)#  
match metric <med-value>  
An update message matches this route map entry if the MED attribute in the update equals  
the value specified in the entry.  
Configuring a set clause  
If a packet matches the match clause, then the action of the route map entry will be applied  
to that packet. The action might simply be to permit or deny the packet, or it might be to  
update its attributes by applying one or more set clauses.  
Note: When configuring a set clause, make sure you are in route map mode for the same  
route map name sequence number as you used for the match clause. The prompt  
should look like this: awplus(config-route-map)#  
A set clause can change any of the following attributes on an update message:  
AS path Multiple Autonomous System numbers (ASNs) can be added to the AS path attribute in the  
update packet. Use the command:  
set as-path prepend <asn-list>  
If adding multiple ASNs, separate them with spaces.  
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BGP: Configuring Route Maps  
Community Community set clauses give you a lot of control over the community values in updates.  
You can:  
Replace the set of community values in an update, by using the command:  
set community <community-values>  
Add more communities to the set of community values in an update, by using the  
command:  
set community <community-values> additive  
Remove the community attribute from an update, by using the command:  
set community none  
Remove communities from the set of community values in an update, by creating a  
community list (see "Configuring a match clause" on page 27) and then applying it in a set  
clause. Use the command:  
set comm-list <list-id>  
Localpref This specifies a value to set as the Localpref attribute in the update message.  
Use the command:  
set local-preference <0-4294967295>  
Metric (MED) This specifies a value to set as the MED attribute in the update message. You can:  
Set the MED in an update, by using the command:  
set metric <0-4294967295>  
Increase or decrease the MED in an update by a specified amount, by using one of the  
commands:  
set metric +<amount>  
set metric -<amount>  
For example, to increase the MED by 2, use the command:  
set metric +2  
Origin This specifies a value to set as the Origin attribute in the update message.  
Use the command:  
set origin {igp|egp|incomplete}  
Aggregator This specifies the update’s aggregator attribute. The aggregator attribute specifies the AS and  
IP address of the device that performed the aggregation.  
Use the command:  
set aggregator <asn> <ipadd>  
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BGP: Configuring Route Maps  
Atomic- This adds the atomic aggregate attribute to the update.  
aggregate  
Use the command:  
set atomic-aggregate  
Extended This specifies a value to set as the Extended Community attribute in the update message.  
community  
Use the command:  
set extcommunity {rt|soo} <ext-comm-number>  
The rt parameter configures a route target extended community. This consists of routers  
that will receive matching routes.  
The soo parameter configures a site-of-origin extended community. This consists of routers  
that will inject matching routes into BGP.  
Next hop This specifies the next hop for matching routes.  
Use the command:  
set ip next-hop <ipadd>  
Originator ID This specifies the Originator ID attribute in the update message. The originator ID is the  
router ID of the IBGP peer that first learned this route, either via an EBGP peer or by some  
other means such as importing it.  
Use the command:  
set originator-id <ipadd>  
Weight Specifies a weight value for matching routes. The weight value assists in best path selection of  
BGP routes. It is stored with the route in the BGP routing table, but is not advertised to  
peers. When there are multiple routes with a common destination, the device uses the route  
with the highest weight value.  
Use the command:  
set weight <0-4294967295>  
Dampening This is used to turn out BGP dampening and optionally set dampening parameters for the  
routes that match the route map entry.  
Use one of the commands:  
set dampening  
set dampening <reachtime>  
set dampening <reachtime> <reuse> <suppress> <maxsuppress>  
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BGP: Configuring Route Maps  
The effect of different combinations of clauses  
A map entry could consist of:  
one match clause with an action, or  
no match clause and one or more set clauses, or  
one match clause and one or more set clauses  
Let us consider each entry type in turn.  
One match clause with an action  
The effect of an entry that contains a match, but no sets, will be to apply the specified action  
to all update messages that match the entry. The action can be either permit or deny. This is  
simply a filter—it simply lets updates through or blocks them; it does not alter updates in any  
way.  
If the match criterion in the match clause specifies AS path or Community and the action is  
deny, then whole update messages that match the criterion will be discarded.  
If the match criterion in the match clause specifies a prefix list or ACL, then only the prefixes  
which match the prefix-list or ACL will be dropped. Those prefixes will be removed from the  
update message, but other prefixes in the update message will not be removed.  
No match clause and one or more set clauses  
If an entry has no match clause, then it is assumed to match all update packets.  
It is possible to add one set clause for each type of attribute that can be set. Of course, it is  
not compulsory to add a clause for each of these attributes. If there is no set clause for an  
attribute, the switch will simply leave it unaltered in update packets.  
It is possible to specify a route map action of deny for a route map that includes a set clause,  
but then there is no point in having defined any attributes to be set, as the routes are going  
to be discarded anyway. However, this is a neat way of turning a clause on or off without  
destroying the configuration entirely.  
Note: Once you have created an entry that has no match clause, there is no point in adding  
any more entries below that one, as this entry will match all packets, and the entries  
below it will never be considered.  
A match clause and one or more set clauses  
The most general case is for an entry to have one match clause (it can never have more than  
one match clause) and a number of set clauses.  
Obviously enough, such an entry will apply the activities defined by the set clauses to only  
those update packets or prefixes that are picked out by the match clause.  
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BGP: Configuring Route Maps  
Particular mention, though, has to be made of the case where the match clause specifies  
prefix list or ACL as the match criterion, and the route map is being applied to outgoing  
route updates.  
The intention of such an entry would be that the attribute values specified in the set clauses  
be applied to only those routes that are contained in the prefix list or ACL specified in the  
match clause.  
What happens when there is an outgoing update message that contains several prefixes,  
some of which are selected by the match clause and some of which are not? Given that an  
update packet contains just one set of attributes, it is not possible to say “change some of the  
attributes in the packet, but make them apply to only some of the prefixes contained in the  
packet”. The attributes in an update packet must apply to all the prefixes in the packet.  
The only solution is to break up the update packet. The switch creates one packet that  
contains the original attribute values and the prefixes that were not specified in the match  
clause of the route map entry. Then the switch creates another packet that contains those  
prefixes of the original packet which did match the prefix list or ACL in the match clause, but  
with modified attribute values.  
The following diagram illustrates the BGP process of splitting an outgoing update:  
BGP process of splitting an outgoing update when a filter changes the AS attribute for some of the prefixes in the update  
Update from Peer 1  
ASPathList=4,3,2,1  
192.168.1.0/24  
Router (AS 5)  
192.168.2.0/24  
172.30.0.0/16  
Add peer Update to IP table  
172.31.0.0/16  
Router generates a new Update, from  
the BGP route table, to send to another  
peer  
Update 1 to Peer 2  
ASPathList = 5,8,4,3,2,1  
192.168.1.0/24  
192.168.2.0/24  
Take internally generated Update and  
add the AS “8” for matches of  
192.168.0.0/16  
Update 2 to Peer 2  
ASPathList = 5,4,3,2,1  
172.30.0.0/16  
172.31.0.0/16  
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BGP: Applying Distribute, Path, Prefix, and Route Map Filters to a Peer  
BGP: Applying Distribute, Path, Prefix, and Route  
Map Filters to a Peer  
Distribute filters, path filters, prefix filters, and route maps can all be applied to a BGP peer  
configuration for both incoming and outgoing updates. However, you cannot combine  
distribute filters (ACLs) and prefix filters.  
First, enter BGP router mode for the AS. The prompt should look like:  
awplus(config-router)#  
Then use the following commands to apply:  
an ACL to filter incoming route updates:  
neighbor <ipadd> distribute-list <acl> in  
a path list to filter incoming route updates:  
neighbor <ipadd> filter-list <list-name> in  
a prefix list to filter incoming route updates:  
neighbor <ipadd> prefix-list <list-name> in  
a route map to filter incoming route updates or alter their attributes:  
neighbor <ipadd> route-map <map-name> in  
an ACL to filter outgoing route updates:  
neighbor <ipadd> distribute-list <acl> out  
a path list to filter outgoing route updates:  
neighbor <ipadd> filter-list <list-name> out  
a prefix list to filter outgoing route updates:  
neighbor <ipadd> prefix-list <list-name> out  
a route map to filter outgoing route updates or alter their attributes:  
neighbor <ipadd> route-map <map-name> out  
You can also use an AS path list, prefix list or ACL in a route map, instead of applying it  
directly as a filter.  
As mentioned in "Hierarchy of the Different Filters" on page 8, if you configure multiple types  
of filter, the switch applies the route maps last. This is true in both the incoming and outgoing  
directions.  
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Examples  
Examples  
Example A A route map that matches on a prefix-list and sets the route metrics  
1. Create a prefix list that matches just 52.0.0.0/8  
awplus(config)#ip prefix-list test1 permit 52.0.0.0/8  
2. Then, create a route map to match on this prefix-list, and set the metric of matching routes  
to 665:  
awplus(config)#route-map test1 permit 1  
awplus(config-route-map)#match ip address prefix-list test1  
awplus(config-route-map)#set metric 665  
3. Configure the route-map to operate on incoming updates from neighbor 45.45.45.46  
awplus(config-router)#neighbor 45.45.45.46 route-map test1 in  
Example B Match on a prefix-list that denies an entry  
1. Create a prefix list that excludes 52.0.0.0/8, then includes all other routes:  
awplus(config)#ip prefix-list test2 deny 52.0.0.0/8  
awplus(config)#ip prefix-list test2 permit any  
2. Create a route map to match on this prefix-list:  
awplus(config)#route-map test2 permit 1  
awplus(config-route-map)#match ip address prefix-list test2  
awplus(config-route-map)#set metric 665  
The route map is:  
route-map test2, permit, sequence 1  
Match clauses:  
ip address prefix-list test2  
Set clauses:  
metric 665  
3. Shut down the neighbor, and then bring it up again:  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
4. Check the route table. This time, the only route learnt from the peer is 45.0.0.0/8, and it  
still has metric 665. The prefix-list has explicitly matched all routes except 52.0.0.0/8, so  
all routes except 52.0.0.0/8 have been permitted by the first entry of the route map. Then  
52.0.0.0/8 has been denied by the implicit deny-all entry at the end of the route map.  
awplus#show ip route  
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Examples  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
C
45.0.0.0/8 [20/665] via 45.45.45.46, vlan1, 00:00:02  
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
Example C Match on a community list, and apply to a deny entry of a route map  
1. Create a community list that has no community types specified:  
ip community-list 1 permit  
The updates from a BGP peer switch will match this community list, if they have no  
community attribute.  
2. Create an IP access-list that matches ALL routes:  
access-list 1 permit any  
3. Create a route map that has two clauses: one which denies all updates matching the  
community list, and one which permits all else.  
route-map com-deny-test deny 1  
match community 1  
route-map com-deny-test permit 2  
match ip address 1  
4. Apply this route map as the in route map on the neighbor 45.45.45.46  
awplus(config-router)# neighbor 45.45.45.46 route-map com-deny-  
test in  
5. Shut down the neighbor, and then bring it up again:  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
6. The routes from the neighbor are now denied, as they match the community-list, and  
therefore the deny entry in the route map:  
awplus#show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
C
C
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
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Examples  
Example D Matching on a next-hop prefix-list  
1. Create a prefix-list that matches on the neighbor's IP address:  
awplus#ship prefix-list nh-test permit 45.45.45.46/32  
2. Create a route map with a permit entry that matches on this prefix-list as a next-hop:  
awplus#shroute-map nh-test permit 1  
awplus#shmatch ip next-hop prefix-list nh-test  
3. Apply this route map as the in route map on the neighbor:  
awplus(config)#router bgp 34567  
awplus(config-router)# neighbor 45.45.45.46 route-map nh-test in  
4. Shut down the neighbor, and then bring it up again:  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
5. All the routes are learnt OK from the neighbor:  
awplus#show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
B
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:00:03  
45.45.45.0/24 is directly connected, vlan1  
52.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:00:03  
64.0.0.0/4 is directly connected, vlan64  
Example E Prepending AS numbers  
1. Create a route map that prepends a list of AS numbers to the attribute field of outgoing  
updates:  
awplus(config)#route-map as-list-test permit 1  
awplus(config-route-map)#match ip address 1  
awplus(config-route-map)#set as-path prepend 11 22 33 44 55 66 77  
88 99 1010 1111 1212 1313 1414 1515 1616  
2. Set this as the out route map for the neighbor 45.45.45.46  
awplus(config-route-map)#router bgp 34567  
awplus(config-router)# neighbor 45.45.45.46 route-map as-list-  
test out  
3. Shut down the neighbor, and then bring it up again:  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
4. Now, check the routes learnt on the neighbor - note the long AS-path list:  
Manager BGP peer> show bgp route  
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Examples  
BGP route table  
Flags: >=Best route for the given prefix, *=Unreachable next hop,  
W=Withdrawn  
m=Community, a=Aggregate route, s=Aggregate Suppressed, D=Damped  
Learned from: L=Local, e=eBGP Peer, i=iBGP Peer, c=Confederate  
Peer  
---------------------------------------------------------------------  
Fl Prefix  
Path  
Next hop  
Origin  
MED  
Local pref  
Originator  
Cluster List  
---------------------------------------------------------------------  
<SNIP>  
> 64.0.0.0/4  
45.45.45.45  
INCOMPLETE -  
100  
SEQ 34567 11 22 33 44 55 66 77 88 99 1010 1111 1212 1313 1414 1515  
1616;  
e -  
-
---------------------------------------------------------------------  
Example G A community-list based filter dropping incoming updates based on community  
number  
1. Configure the AW peer to send out a community number 89:89  
add ip routem=com entry=1 set commmun=89:89  
set bgp peer=45.45.45.45 outroutemap=com sendcommunity=yes  
The routes coming from that peer has community 89:89  
BGP#show ip bgp 52.0.0.0/8  
BGP routing table entry for 52.0.0.0/8  
Paths: (1 available, best #1, table Default-IP-Routing-Table)  
Not advertised to any peer  
34568  
45.45.45.46 from 45.45.45.46 (45.45.45.46)  
Origin IGP metric 0, localpref 100, valid, external, best  
Community: 89:89 ----------------------------  
Last update: Wed Oct 1 06:47:01 2008  
1. Then, on the AlliedWare Plus switch, create a community-list that matches on some  
community values.  
ip community-list 78 permit 55:66  
ip community-list 78 permit 89:89  
ip community-list 78 permit 9999:89  
2. Create a prefix-list that matches the route 52.0.0.0/8  
ip prefix-list test1 permit 52.0.0.0/8  
3. Set the community-list to be the match criterion on a deny entry in a route map  
route-map com deny 1  
match community 78  
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Examples  
4. Set the prefix-list to be the match criterion on a permit entry in the route map:  
route-map com permit 2  
match ip address prefix-list test1  
If the route 52.0.0.0/8 is dropped by this route map, we can be sure that it was dropped by  
the first deny entry, and not by the implicit deny-all entry at the end of the route map.  
5. Apply this route map as the incoming filter for the neighbor 45.45.45.46:  
neighbor 45.45.45.46 route-map com in  
6. Shut down the neighbor, and then bring it up again:  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
If the deny clause in the routemap does its job, then the routes advertised from the peer will  
be dropped. And, indeed they are dropped.  
BGP#show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
C
C
45.45.45.0/24 is directly connected, vlan1  
64.0.0.0/4 is directly connected, vlan64  
Example H Combination of a distribute list and a route map and an AS-Path list filter  
1. Add some new static routes to the AW peer, and configure the peer to redistribute these  
static routes into BGP:  
add ip rou=156.23.4.0 mask=255.255.255.248 int=vlan2  
next=52.34.5.4  
add ip rou=156.23.4.8 mask=255.255.255.248 int=vlan2  
next=52.45.45.45  
add ip rou=156.23.4.32 mask=255.255.255.224 int=vlan2  
next=52.45.45.45  
add ip rou=156.23.4.144 mask=255.255.255.240 int=vlan2  
next=52.45.45.45  
add bgp import=static  
2. Configure the peer to give Community 89:89 to the route 156.23.4.32/27 and append AS  
path 34599 to the route 156.34.4.144/28.  
add ip prefixlist=com entry=1 action=match prefix=156.23.4.32  
masklength=27  
add ip prefixlist=as entry=1 action=match prefix=156.23.4.144  
masklength=28  
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Examples  
add ip routem=mixed entry=1 match prefixlist=com  
add ip routem=mixed entry=1 set com=89:89  
add ip routem=mixed entry=2 match prefixlist=as  
add ip routem=mixed entry=2 set aspath=34599  
set bgp peer=45.45.45.45 outroutemap=mixed sendcommunity=yes  
When the route 156.23.4.144/28 is checked on the AlliedWare Plus switch, its ASPath  
contains 34599.  
BGP#show ip bgp 156.23.4.144/28  
BGP routing table entry for 156.23.4.144/28  
Paths: (1 available, best #1, table Default-IP-Routing-Table)  
Not advertised to any peer  
34568 34599  
45.45.45.46 from 45.45.45.46 (45.45.45.46)  
Origin incomplete metric 0, localpref 100, valid, external, best  
Last update: Wed Oct 1 08:54:10 2008  
3. On the AlliedWare Plus switch, create an ASPath-list that drops AS paths containing  
34599, but allows all others.  
ip as-path access-list list1 deny 34599  
ip as-path access-list list1 permit .*  
4. Create a community-list that includes the community 89:89, among others.  
ip community-list 78 permit 55:66  
ip community-list 78 permit 89:89  
ip community-list 78 permit 9999:89  
5. Create a route map that drops updates that match this community-list, but allows all other  
updates.  
route-map com deny 1  
match community 78  
route-map com permit 2  
6. Apply both the aspath-list and the route map as in-filters on the neighbor:  
router bgp 34567  
redistribute connected  
neighbor 45.45.45.46 remote-as 34568  
neighbor 45.45.45.46 route-map com in  
neighbor 45.45.45.46 filter-list list1 in  
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Examples  
With this combination, neither 156.23.4.32/27 nor 156.34.4.144/28 appear in the IP route  
table. The route 156.23.4.32/27 is dropped by the route map filter, and the route  
156.34.4.144/28 is dropped by the ASPath-list filter.  
BGP#show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
B
C
B
B
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:06:39  
45.45.45.0/24 is directly connected, vlan1  
52.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:06:39  
64.0.0.0/4 is directly connected, vlan64  
156.23.4.0/29 [20/0] via 45.45.45.46, vlan1, 00:06:39  
156.23.4.8/29 [20/0] via 45.45.45.46, vlan1, 00:06:39  
7. Now create an ACL that drops the route 156.23.4.0/28, and allows all others.  
access-list standard list3 deny 156.23.4.0/28  
access-list standard list3 permit any  
8. Add that ACL as a distribute-list in-filter on the neighbor:  
router bgp 34567  
redistribute connected  
neighbor 45.45.45.46 remote-as 34568  
neighbor 45.45.45.46 distribute-list list3 in  
neighbor 45.45.45.46 route-map com in  
neighbor 45.45.45.46 filter-list list1 in  
9. Shut down the neighbor, and then bring it up again:  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
Then all the 156.23.4.x routes are filtered out. The distribute-list filter drops both the  
routes 156.23.4.0/29 and 156.23.4.8/29 because the effect of the distribute-list filter is to  
drop all routes within the address range covered by 156.23.4.0/28, that have a prefix-  
length of 28 or longer.  
BGP#show ip route  
Codes: C - connected, S - static, R - RIP, B - BGP  
O - OSPF, IA - OSPF inter area  
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2  
E1 - OSPF external type 1, E2 - OSPF external type 2  
* - candidate default  
B
C
B
C
45.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:16:21  
45.45.45.0/24 is directly connected, vlan1  
52.0.0.0/8 [20/0] via 45.45.45.46, vlan1, 00:16:21  
64.0.0.0/4 is directly connected, vlan64  
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Examples  
Example I Using an ACL-match in a route map to update just a single route out of an update  
1. Configure static routes on the AW+ switch.  
BGP(config)#ip route 192.34.23.0/26 64.93.23.1  
BGP(config)#ip route 192.34.23.32/29 64.193.54.1  
BGP(config)#ip route 192.34.23.192/27 64.12.89.1  
2. Configure BGP to redistribute static routes.  
router bgp 34567  
redistribute connected  
redistribute static  
neighbor 45.45.45.46 remote-as 34568  
3. Create an ACL that matches just one of these routes.  
access-list standard marker permit 192.34.23.32/29  
4. Create a route map with one clause that matches on this ACL and prepends an AS-path  
(and then has another clause the permits everything else).  
route-map marker permit 1  
match ip address marker  
set as-path prepend 7822  
route-map marker permit 2  
5. Apply this route map as the out route map for the peer.  
router bgp 34567  
redistribute connected  
redistribute static  
neighbor 45.45.45.46 remote-as 34568  
neighbor 45.45.45.46 route-map marker out  
6. Shut down the neighbor, and then bring it up again:  
awplus(config-router)# neighbor 45.45.45.46 shutdown  
awplus(config-router)# neighbor 45.45.45.46 no shutdown  
7. Check the BGP table on the peer. Only one route should arrive at the other end with that  
extra entry in the as-path.  
Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes | Page 43  
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Examples  
BGP route table  
Flags: >=Best route for the given prefix, *=Unreachable next hop,  
W=Withdrawn  
m=Community, a=Aggregate route, s=Aggregate Suppressed, D=Damped  
Learned from: L=Local, e=eBGP Peer, i=iBGP Peer, c=Confederate  
Peer  
---------------------------------------------------------------------  
Fl Prefix  
Path  
Next hop  
Origin  
MED  
Local pref  
Originator  
Cluster List  
---------------------------------------------------------------------  
> 45.0.0.0/8  
EMPTY  
L -  
> 45.45.45.0/24  
SEQ 34567;  
e -  
> 52.0.0.0/8  
EMPTY  
0.0.0.0  
IGP  
-
100  
100  
100  
100  
100  
100  
100  
100  
100  
100  
-
45.45.45.45  
INCOMPLETE -  
-
0.0.0.0  
IGP  
-
L -  
-
> 64.0.0.0/4  
SEQ 34567;  
e -  
> 156.23.4.0/29  
EMPTY  
45.45.45.45  
INCOMPLETE -  
INCOMPLETE -  
INCOMPLETE -  
INCOMPLETE -  
INCOMPLETE -  
INCOMPLETE -  
INCOMPLETE -  
-
52.34.5.4  
L -  
-
> 156.23.4.8/29  
EMPTY  
L -  
> 156.23.4.32/27  
EMPTY  
L -  
> 156.23.4.144/28  
EMPTY  
L -  
> 192.34.23.0/26  
SEQ 34567;  
e -  
52.45.45.45  
-
52.45.45.45  
-
52.45.45.45  
-
45.45.45.45  
-
> 192.34.23.32/29  
45.45.45.45  
SEQ 34567 7822; -------------------------------------------------  
e -  
-
> 192.34.23.192/27  
SEQ 34567;  
e -  
45.45.45.45  
INCOMPLETE -  
100  
-
---------------------------------------------------------------------  
Page 44 | Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes  
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Ways to use lists in IP route filtering for BGP, with generic command examples  
ACLs  
bgp neighbor x.x.x.x distribute-list 1 in  
bgp neighbor x.x.x.x filter-list <alist1> in  
access-list 1 deny x.x.x.x 0.0.0.255  
Path filters  
ip as-path access-list <aslist1> ...  
... permit <reg-ex>  
... deny <reg-ex>  
... permit <reg-ex>  
ip as-path access-list <aslist2> ...  
... permit <reg-ex>  
bgp neighbor x.x.x.x filter-list <alist2> out  
... deny <reg-ex>  
Prefix filters  
ip prefix-list <plist1> ...  
... permit x.x.x.x/24  
... deny x.x.x.x/24  
bgp neighbor x.x.x.x prefix-list <plist1> in  
bgp neighbor x.x.x.x prefix-list <plist2> out  
ip prefix-list <plist2> ...  
... permit x.x.x.x/24  
... deny x.x.x.x/24  
Community  
lists  
ip community-list <clist1> ...  
... deny internet  
... permit noexport  
Route maps  
Entities in the column above are  
the possible lists that can be a part  
of per-peer BGP filtering. These lists  
can be applied directly to a BGP  
peer (except for community lists) or  
added to a route map (middle  
column), which is then added to a  
BGP peer (last column).  
route-map <map1> permit 1  
match ip address 1  
bgp neighbor x.x.x.x route-map <map1> out  
bgp neighbor x.x.x.x route-map <map2> in  
route-map <map2> deny 1  
match as-path <alist2>  
route-map <map3> permit 1  
match ip address prefix-list <plist2>  
bgp neighbor x.x.x.x route-map <map3> in  
bgp neighbor x.x.x.x route-map <map4> out  
ip route-map <map4> permit 1  
match community <clist1>  
set comm-list <clist1> delete  
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BGP: Applying Route Maps to Imported Routes  
BGP: Applying Route Maps to Imported Routes  
The switch is able to import routes into BGP that it learnt by non-BGP means. In other  
words, the routes would be static routes, or routes learnt by OSPF or RIP.  
You can apply a route map to this importation process so that the imported routes are given  
certain attributes, or so that certain routes are blocked from being imported.  
The switch uses the route map to:  
filter routes or set attributes when it imports the routes into BGP  
set attributes on any update message in which it advertises the routes  
Note that the entries in route maps that are applied to BGP importing cannot have match  
clauses that match on BGP attributes such as AS path or community. This is because BGP  
attributes are not relevant to non-BGP routes. The route map entries can match on prefix  
list, origin, next hop, or the route type if importing OSPF routes.  
Syntax  
You can apply the route map by using the following command in BGP router mode for the  
AS.  
redistribute {connected|ospf|rip|static} route-map <map-name>  
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Other Uses of Route Maps  
Other Uses of Route Maps  
Route maps are used in some contexts other than filtering routes. Let us look briefly at some  
of the other contexts in which they are used.  
neighbor default-originate  
The command neighbor default-originate instructs BGP to send a default route to a neighbor.  
This command includes a parameter for specifying a route map.  
The route map parameter specifies criteria that must be fulfilled before the switch will  
advertise the default route to the neighbor. This lets you configure the switch so that it only  
acts as the default gateway for a neighbor if it has learned a route that makes it a suitable  
default gateway for that neighbor.  
Specifically, the switch must have learnt a route that matches the permit criteria in the route  
map. If the switch does not know of any routes that match the permit criteria in the route  
map, then it will not advertise the default route.  
neighbor unsuppress-map  
When the aggregate-address command is used with the summary-only option, the more  
specific routes of the aggregate are suppressed to all neighbors. If you want to selectively leak  
some more-specific routes to a particular neighbor, then this is achieved by using the  
command neighbor unsuppress-map.  
A required parameter on this command is the route map that specifies the set of more-  
specific routes to leak. Any route that matches permit criteria in the route map will be  
leaked.  
network  
The network command is used in BGP to selectively redistribute particular routes. Instead of  
saying “redistribute all static routes” or “redistribute all connected routes”, the network  
command says “redistribute this particular route”. Of course, the route has to be in the  
routing table before it can be redistributed, so the network command does not say “always  
import this route in BGP”, it says “if this route is in the routing table, then import it into  
BGP”.  
As with other acts of redistribution, the network command has an optional route map  
parameter. This lets you set attributes on the route advertised by the network command.  
You could use the route map for filtering the network, but it would be a little pointless to  
even configure the redistribution of this network if the associated route map simply dropped  
the route.  
show ip bgp  
The show ip bgp command takes an optional route map parameter. The effect of specifying  
the route map on the command is that the output shows information only on routes that  
match permit criteria on the route map.  
Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes | Page 47  
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BGP: Route Map Filtering Example  
BGP: Route Map Filtering Example  
Here is an example of a set of route maps.  
BGP configuration  
First, we need to set the router’s ASN and the ASN of the peer.  
router bgp 3816  
neighbor 172.26.1.1 remote-as 15557  
neighbors are enabled by default.  
Route map configuration  
Next, we want to limit the routes that we accept from this peer. We will accept the default  
route from any community, but will only accept any other updates from community 15557.  
ip prefix-list plist1 seq 1 permit 0.0.0.0/0  
ip community-list 1 permit 15557  
route-map accDef15557 permit 1  
match ip address prefix-list plist1  
route-map accDef15557 permit 2  
match community 1 exact-match  
router bgp 3816  
neighbor 172.26.1.1 route-map accDef15557 in  
Next, we want to change the attributes of certain routes before advertising them to the  
neighbor 172.26.1.v. For 192.168.2.0/24, we will add to the AS path. For 192.168.3.0/24 and  
192.168.4.0/24, we will add a different set of entries to the AS path, and set the community  
and the local preference. All other routes will be advertised unchanged this neighbor.  
ip prefix-list plist2 seq 1 permit 192.168.2.0/24  
ip prefix-list plist3 seq 1 permit 192.168.3.0/24  
ip prefix-list plist4 seq 1 permit 192.168.4.0/24  
route-map outdef permit 1  
match ip address prefix-list plist2  
set as-path prepend 1645  
route-map outdef permit 2  
match ip address prefix-list plist3  
set as-path prepend 65501 63901 43312  
set community 0:23445 0:33445 no-advertise  
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BGP: Route Map Filtering Example  
set local-preference 9000  
route-map outdef permit 3  
match ip address prefix-list plist4  
set as-path prepend 64751 1827  
set community no-export additive  
set local-preference 9000  
route-map outdef permit 4  
router bgp 3816  
neighbor 172.26.1.1 route-map outdef out  
Finally, we want to import the host-specific prefix 192.168.200.200/32 into BGP, and set a  
community value on that entry.  
route-map netspec permit 1  
set community 8888:8888 9999:9999  
router bgp 3816  
network 192.168.200.200/32 route-map netspec  
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OSPF: Configuring Route Maps for Filtering and Modifying OSPF Routes  
OSPF: Configuring Route Maps for Filtering and  
Modifying OSPF Routes  
For information about route maps and their structure, see "Structure of a route map" on  
Route maps can be applied to OSPF routes as well as BGP routes. Of course, the route  
maps that can be used with OSPF are rather limited in comparison with those that are used  
with BGP, as OSPF route updates do not carry attributes in the way that BGP route updates  
do.  
Also, OSPF route maps can only be applied to importing:  
OSPF-learnt routes into the main IP route table, or  
static, BGP or RIP routes into OSPF  
Filtering cannot:  
remove an entry from the LSA database once the entry has been added  
prevent the router from advertising an LSA database entry to interfaces in the same area  
that the entry is relevant to  
prevent updates that OSPF learns from being put into the LSA database  
change the properties of an entry in the LSA database  
This is because OSPF shares LSAs between all the routers in an area. The protocol assumes  
that all the routers in the area have shared all the advertisements among each other, and that  
all agree on the state of the complete link state database for the area. If some routers in the  
area are learning, but not advertising, that breaks the OSPF model.  
Configuring a match clause  
A match clause can match on the following criteria.  
Note: When configuring a match clause, make sure you are in route map mode.  
The prompt should look like: awplus(config-route-map)#  
Metric  
The entry will match all routes whose metric is equal to that specified in the clause.  
To match a metric value, use the command:  
match metric <value>  
Interface  
The entry will match all routes learnt via the specified VLAN.  
To match a VLAN, use the command:  
match interface <vlan>  
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OSPF: Configuring Route Maps for Filtering and Modifying OSPF Routes  
External route type  
The entry will match all routes of either Type 1 External or Type 2 External.  
To match a route type, use the command:  
match external {type-1|type-2}  
A prefix, by using a prefix list  
The entry will match one or more route prefixes.  
For information about creating a prefix list, see "About prefix lists" on page 22.  
Once you have made the prefix list, apply it to the match clause of a route map entry by  
using the command:  
match ip address prefix-list <list-name>  
A prefix, by using an ACL  
An ACL is an alternative to a prefix list for matching route prefixes.  
For information about creating an ACL, see "About ACLs" on page 11.  
Once you have made the ACL, apply it to the match clause of a route map entry by using the  
command:  
match ip address <acl-number-or-name>  
A next hop address  
The entry will match the route’s next hop.  
You can use either a prefix list or an ACL to specify a next hop address. Once you have made  
the prefix list or ACL, apply it to the match clause of a route map entry by using one of the  
commands:  
match ip next-hop prefix-list <list-name>  
match ip next-hop <acl-number-or-name>  
Configuring a set clause  
If a route matches the match clause, then the action of the route map entry will be applied  
to that route. The action might simply be to permit or deny the route, or it might be to  
update its parameters by applying one or more set clauses.  
Note: When configuring a set clause, make sure you are in route map mode for the same  
route map name sequence number as you used for the match clause.  
The prompt should look like: awplus(config-route-map)#  
A set clause can alter the following parameters on a route.  
Use Route Maps and Other Filters to Filter and Alter BGP and OSPF Routes | Page 51  
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Metric This changes the route metric. You can:  
Set the metric, by using the command:  
set metric <0-4294967295>  
Increase or decrease the metric by a specified amount, by using one of the commands:  
set metric +<amount>  
set metric -<amount>  
For example, to increase the metric by 2, use the command:  
set metric +2  
Next hop This specifies the next hop for matching routes.  
Use the command:  
set ip next-hop <ipadd>  
Type This sets the route type to either Type 1 External or Type 2 External.  
Use the command:  
set metric-type {type-1|type-2}  
OSPF: Applying Route Maps  
To specify a route map to be applied to static, BGP, RIP or connected routes as they are  
imported to OSPF, use the commands:  
router ospf  
redistribute {bgp|rip|connected|static} route-map <map-name>  
Note that if you want to filter OSPF routes as they are imported into the main IP route table,  
you need to use a distribute filter instead of a route map.  
Use commands like the following:  
router ospf 88  
distribute-list list1 in  
C613-16128-00 REV E  
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