BGP Route Advertisement | Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)

Some networks may not be advertised properly, which results in their non-reachability from networks connected to BGP peers. Consider the example shown in Figure 11.4 to understand and troubleshoot this scenario.

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Figure 11.4: A BGP network depicting a BGP route advertisement problem.

In Figure 11.4, BGP is configured between AS6666 and AS7777. Routers A1 and B1 are BGP peers. A1 and A2 in AS6666 are connected by a serial link with static routing configured. Consider the scenario in which network 192.168.20.0/24 is not getting advertised to AS7777. The command A1#show ip bgp neighbor 10.1.1.1 advertised-routes | include 192.168.20 will yield no output. This confirms that the route is not getting advertised.

To troubleshoot this router problem:

Check if the route is configured in BGP by using network command. The network command should be present in the configuration of A1. It will be shown as network 192.168.20.0. If the command is not available, the network will not be advertised.
Check for the actual presence of the route in the routing table of A1 by using the show ip route command. The output for the show ip route command is as shown in Listing 11.16.

Listing 11.16 Output of show ip route Command
```
A1#show ip route 192.168.20.0 Routing entry for 192.168.20.0/24   Known via "static", distance 1, metric 0   Routing Descriptor Blocks:   * 10.1.2.2       Route metric is 0, traffic share count is 1      
```
In Listing 11.16, the route is shown as available. If the route is not available in the routing table, it would not be advertised.

Check if the route is part of the BGP table at A1 by using the show ip bgp command. The output of the show ip bgp command is shown in Listing 11.17.

Listing 11.17 Output of show ip bgp Command

A1#show ip bgp 192.168.20.0 BGP routing table entry for 192.168.20.0/24, version 5703 Paths: (1 available, best #1, table Default-IP-Routing Table)   Advertised to non peer-group peers:   10.1.1.1 Local     0.0.0.0 from 0.0.0.0 (10.1.2.1)       Origin IGP, metric 0, localpref 100, weight 32768, valid,  sourced, local, best

In Listing 11.17, the route is found to be part of the BGP table at A1.

Check the BGP configuration of BGP peers for any filtering implemented. The relevant part of the BGP configuration of A1 is shown in Listing 11.18.

Listing 11.18 Configuration of A1

router bgp 6666 neighbor  10.1.1.1 remote-as 7777 neighbor  10.1.1.1 route-map NEI out ! route-map NEI permit 10 match ip address 20 set metric 100 ! access-list 20 192.168.0.0 0.0.255.255 access-list 20 permit any

Listing 11.18 shows that the route map NEI has been applied for updates to the BGP peer B1 at A1, which matches routes against access list 20. Access list 20 denies the network 192.168.10.0/24. This causes the nonadvertisement of the route. Suitable modification of the access list will result in the successful advertisement of the route.