Lab 127 - BGP Allowas-In

Prerequisites: CCNP level skills.

Topology

Pic. 1 - Topology Diagram.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1
R4 and R5 are the edge routers of AS 45. Configure AS 123 in such a way, that the prefixes advertised by R4 are accepted by BGP table on R5 and vice versa.

Solution

Task 1
R4 and R5 are the edge routers of AS 45. Configure AS 123 in such a way, that the prefixes advertised by R4 are accepted by BGP table on R5 and vice versa.

Notice!
BGP routers use eBGP loop prevention mechanism which prompts them to check if their own AS is anywhere in the path. If so, the prefix is rejected (looped). In corner case scenarios like this, we must disable this mechanism (this may cause loops so care must be taken when configuring this).

Pic. 2 - R5's Peering with R3.

Pic. 3 - BGP Table on R5.

Pic. 4 - Update Rejected on R5.

Notice!

Output truncated (all prefixes advertised into BGP by R4 are rejected, R4 rejects the prefix advertised by R5)

R4 Configuration:

!

router bgp 45
 no synchronization
 bgp router-id 172.16.104.4
 bgp log-neighbor-changes
 network 172.16.104.0 mask 255.255.255.0
 network 172.16.144.0 mask 255.255.255.0
 redistribute connected route-map CONN_TO_BGP
 neighbor 10.1.14.1 remote-as 123
 neighbor 10.1.14.1 allowas-in 1
 neighbor 10.1.24.2 remote-as 123
 neighbor 10.1.24.2 allowas-in 1
 no auto-summary

!

R5 Configuration:

!

router bgp 45
 no synchronization
 bgp router-id 172.16.105.5
 bgp log-neighbor-changes
 network 172.16.105.0 mask 255.255.255.0
 neighbor 10.1.35.3 remote-as 123
 neighbor 10.1.35.3 allowas-in 1
 no auto-summary

!

Notice!
The option 'allowas-in 1' allows for its own AS (AS 45) to appear once (1) in the AS_PATH attribute. The prefixes are now accepted.

Pic. 5 - BGP Table on R5.