The local AS community is a well known BGP community and can be used for BGP confederations. It’s basically the same as the no export community but this one works for within the sub-AS of a confederation. Prefixes that are tagged are only advertised to other neighbors in the same sub-AS, not to other sub-AS’es or eBGP routers.

To demonstrate this I will use the following topology:

AS 2345 has 4 routers and 2 sub-AS’es. We will advertise a prefix from R1 to AS 2345 so you can see what happens with and without the use of the local AS community. Let’s look at the configuration…

Configuration

BGP Configuration

Below you will find the BGP configuration for all these routers. Within AS 2345 I have used OSPF so that these routers can use their loopback interfaces for BGP.

R1#show running-config | section bgp
router bgp 1
 no synchronization
 bgp log-neighbor-changes
 network 1.1.1.1 mask 255.255.255.255
 neighbor 192.168.12.2 remote-as 2345
 no auto-summary

R2#show running-config | section bgp
router bgp 23
 no synchronization
 bgp log-neighbor-changes
 bgp confederation identifier 2345
 bgp confederation peers 45 
 neighbor 3.3.3.3 remote-as 23
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 4.4.4.4 remote-as 45
 neighbor 4.4.4.4 ebgp-multihop 2
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 192.168.12.1 remote-as 1
 no auto-summary

R3#show running-config | section bgp
router bgp 23
 no synchronization
 bgp log-neighbor-changes
 bgp confederation identifier 2345
 bgp confederation peers 45 
 neighbor 2.2.2.2 remote-as 23
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 5.5.5.5 remote-as 45
 neighbor 5.5.5.5 ebgp-multihop 2
 neighbor 5.5.5.5 update-source Loopback0
 neighbor 192.168.36.6 remote-as 6
 no auto-summary

R4#show running-config | section bgp
router bgp 45
 no synchronization
 bgp log-neighbor-changes
 bgp confederation identifier 2345
 bgp confederation peers 23 
 neighbor 2.2.2.2 remote-as 23
 neighbor 2.2.2.2 ebgp-multihop 2
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 5.5.5.5 remote-as 45
 neighbor 5.5.5.5 update-source Loopback0
 no auto-summary

R5#show running-config | section bgp
router bgp 45
 no synchronization
 bgp log-neighbor-changes
 bgp confederation identifier 2345
 bgp confederation peers 23 
 neighbor 3.3.3.3 remote-as 23
 neighbor 3.3.3.3 ebgp-multihop 2
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 4.4.4.4 remote-as 45
 neighbor 4.4.4.4 update-source Loopback0
 no auto-summary

R6#show running-config | section bgp
router bgp 6
 no synchronization
 bgp log-neighbor-changes
 neighbor 192.168.36.3 remote-as 2345
 no auto-summary

R1 has advertised prefix 1.1.1.1/32 in BGP, let’s see if our routers have learned this:

R2#show ip bgp | begin 1.1.1.1
*> 1.1.1.1/32       192.168.12.1             0             0 1 i

R3#show ip bgp | begin 1.1.1.1
*>i1.1.1.1/32       192.168.12.1             0    100      0 1 i

R4#show ip bgp | begin 1.1.1.1
* i1.1.1.1/32       192.168.12.1             0    100      0 (23) 1 i
*>                  192.168.12.1             0    100      0 (23) 1 i

R5#show ip bgp | begin 1.1.1.1
* i1.1.1.1/32       192.168.12.1             0    100      0 (23) 1 i
*>                  192.168.12.1             0    100      0 (23) 1 i

R6#show ip bgp | begin 1.1.1.1
*> 1.1.1.1/32       192.168.36.3                           0 2345 1 i

All router know about this prefix. Time to activate the local AS community…

Local AS Community Configuration

We will create a route-map on R2  that sets the local AS community on all prefixes that it receives from R1:

R2(config)#route-map LOCAL_AS permit 10
R2(config-route-map)#set community local-AS 

R2(config)#router bgp 23
R2(config-router)#neighbor 192.168.12.1 route-map LOCAL_AS in
R2(config-router)#neighbor 3.3.3.3 send-community

R2 sets the community so make sure that it advertises it to R3. Before we reset BGP, take a look at the BGP table of R2: