Concept Guide

Filtering on an AS-Path Attribute

You can use the BGP attribute, AS_PATH, to manipulate routing policies.

The AS_PATH attribute contains a sequence of AS numbers representing the route’s path. As the route traverses an AS, the ASN is

prepended to the route. You can manipulate routes based on their AS_PATH to aect interdomain routing. By identifying certain ASN in the

AS_PATH, you can permit or deny routes based on the number in its AS_PATH.

AS-PATH ACLs use regular expressions to search AS_PATH values. AS-PATH ACLs have an “implicit deny.” This means that routes that do

not meet a deny or match lter are dropped.

To congure an AS-PATH ACL to lter a specic AS_PATH value, use these commands in the following sequence.

1 Assign a name to a AS-PATH ACL and enter AS-PATH ACL mode.

CONFIGURATION mode

ip as-path access-list as-path-name

2 Enter the parameter to match BGP AS-PATH for ltering.

CONFIG-AS-PATH mode

{deny | permit} filter parameter

This is the lter that is used to match the AS-path. The entries can be any format, letters, numbers, or regular expressions.

You can enter this command multiple times if multiple lters are desired.

For accepted expressions, refer to Regular Expressions as Filters.

3 Return to CONFIGURATION mode.

AS-PATH ACL mode

exit

4 Enter ROUTER BGP mode.

CONFIGURATION mode

router bgp as-number

5 Use a congured AS-PATH ACL for route ltering and manipulation.

CONFIG-ROUTER-BGP mode

neighbor {ip-address | ipv6-address | peer-group-name} filter-list as-path-name {in | out}

If you assign an non-existent or empty AS-PATH ACL, the software allows all routes.

Example of the show ip bgp paths Command

To view all BGP path attributes in the BGP database, use the show ip bgp paths command in EXEC Privilege mode.

DellEMC#show ip bgp paths

Total 386 Paths

Refcount Metric Path

20 0 64801 i

Border Gateway Protocol (BGP)

223