BGP as-override

Another one:

R1, R5 and R6 are CE routers all belong to the same BGP autonomous system AS65156 while R2, R3 and R4 are service provider PE’s belong to AS65000. All CE’s are connected in MPLS VPN, start topology, to the SP network in VRF named CE1.

All 192.168.xx.0/24 networks which are advertised by R1, R5 and R6 won’t be learned by each other due to AS-PATH loop-prevention mechanism which doesn’t allow BGP peer to learn a prefix with his own AS in the AS-PATH.

We can see from the packet capture that R2 (1) is sending to R1 (2) a BGP update message (3) with AS-PATH 65000 65156 (4) for networks 192.168.61.0/24 and 192.168.51.0/24 (5).

R1 in return discard these networks because he sees his AS number in the AS-PATH.

R1:

R1#show ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, 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

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

C    192.168.11.0/24 is directly connected, Loopback1

     10.0.0.0/24 is subnetted, 3 subnets

C       10.1.12.0 is directly connected, FastEthernet0/0

B       10.1.46.0 [20/0] via 10.1.12.2, 08:07:12

B       10.1.35.0 [20/0] via 10.1.12.2, 08:07:12

Now we can overcome this issue by configuring allow-in in each of the CE routers (Customer side solution) or we can configure as-override on the PE’s (SP solution side) which basiclly takes the customer AS and convert it to his AS, in our case, instruct the PE’s to convert AS65156 to AS65000 for every network prefix they send to their neighbors.

R2:

R2(config)#router bgp 65000

R2(config-router)#address-family ipv4 vrf CE1

R2(config-router-af)#neighbor 10.1.12.1 as-override

The result, as we can see in the pacekt capture:

R2 (1) is sending to R1 (2) a BGP update message (3) with AS-PATH 65000 65000 (4) for networks 192.168.61.0/24 and 192.168.51.0/24 (5).

And now R1 sees his other CE members networks:

R1# show ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, 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

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

B    192.168.61.0/24 [20/0] via 10.1.12.2, 00:04:54

C    192.168.11.0/24 is directly connected, Loopback1

     10.0.0.0/24 is subnetted, 3 subnets

C       10.1.12.0 is directly connected, FastEthernet0/0

B       10.1.46.0 [20/0] via 10.1.12.2, 00:04:54

B       10.1.35.0 [20/0] via 10.1.12.2, 00:04:54

B    192.168.51.0/24 [20/0] via 10.1.12.2, 00:04:54

Note that the as-override command will be avliable only for IPv4 VRF address family neighbors (MP-BGP), we can’t use this feature when working with normal BGP neighbors.