BGP Communities

To discuss the troubleshooting scenario in BGP communities, consider the example shown in Figure 11.10. In the figure, part of a BGP-enabled network is shown.

Figure 11.10: A BGP network depicting the BGP communities’ problem.

BGP is running between router B1 of AS5555 and router A1 of AS6666. iBGP is running within B1 and B2 of AS5555, and A1 and A2 of AS6666.

In Figure 11.10, the network is configured with the help of communities. Consider a scenario in which network 192.168.4.0/24 is not to be advertised to AS6666. It is found that network 192.168.4.0/24, which was to be restricted from being advertised to AS6666, is available in the BGP table of A1 and A2.

To troubleshoot this problem, check if the proper community has been associated with network 192.168.4.0/24. The relevant part of the configuration of B2 is shown in Listing 11.30.

Listing 11.30 Configuration of B1

router bgp 6666 neighbor 10.10.2.1 remote-as 6666 neighbor 10.10.2.1 route-map COMM out ! route-map COMM permit 10 match ip address 15 set community no-export ! access-list 15 permit 192.168.4.0

The community attribute of the route 192.168.4.0/24 needs to be set as no-export to prevent the advertisement to eBGP peers, which is A1 in this case.

The community attribute is not carried by default in BGP updates. In order for it to be carried, the neighbor 10.10.2.1 send-community command must be present in the BGP configuration.