Information Technology Reference
In-Depth Information
When modifying the default BGP behavior, you must understand the full implications to
avoid undesired side effects. As shown in Example 5-8, the prefix 10.1.0.0/16 originates in
AS 65101. R11 receives two paths via BGP, from R3 and R9. Modifying LOCAL_PREF
to incoming paths from R9 causes the traffic destined for AS 65101 to be sent via AS 65103.
Assuming that AS 65104 wants to use the direct connection to AS 65101 via the link
between R3 and R11, this new behavior is undesirable.
Prefix 10.1.0.0/16, Originated in AS 65101
Example 5-8
R11#show ip bgp 10.1.0.0
BGP routing table entry for 10.1.0.0/16, version 22
Paths: (2 available, best #1, table Default-IP-Routing-Table)
Flag: 0x208
Advertised to non peer-group peers:
172.16.13.9
65103 65101
172.16.13.17 from 172.16.13.17 (172.16.9.1)
Origin IGP, localpref 120, valid, external, best
65101
172.16.13.9 from 172.16.13.9 (172.16.3.1)
Origin IGP, metric 307200, localpref 100, valid, external
120
best
100
This problem can be solved by applying an as-path list to the route map to match only
prefixes originated by AS 65102 to modify the local preference. The new route map
configuration is shown in Example 5-9.
BGP and Route Map Configuration on R11
Example 5-9
!
ip as-path access-list 1 permit _65102$
!
route-map LPREF permit 10
match as-path 1
set local-preference 120
!
route-map LPREF permit 20
!
The final result is shown in Example 5-10 for prefix 10.1.0.0/16 and in Example 5-11 for
prefix 10.2.0.0/16.
Example 5-10
Path Information for 10.1.0.0/16
R11#show ip bgp 10.1.0.0
BGP routing table entry for 10.1.0.0/16, version 45
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
172.16.13.17
65103 65101
