Missing Routes

Some destinations may become unreachable due to the absence of BGP routes from the routing table. In this section, you will have a look at a similar scenario with the help of an example and steps to overcome the problem. Figure 11.5 shows BGP running between AS5555 and AS6666.

Figure 11.5: BGP running between A1 and B1 in AS5555 and AS6666.

In Figure 11.5, B1 does not have any route to network 10.10.1.0/24. This is evident from the commands shown:

B1#show ip route 10.10.1.1 % Network not in table

To troubleshoot this missing route problem:

1. Check the existence of the path in the BGP table by using the command:

   B1#show ip bgp 10.10.1.1 % Network not in table

   The command output shows that there is no corresponding entry in the BGP table.

2. Check the BGP neighbor relation establishment with A1. If the neighbor relationship is not established, execute the troubleshooting steps for the same.

3. Check if route 10.10.1.1/24 is being learned from A1 after the neighbor relationship is established by using the command:

   B1#show ip bgp neighbor 192.168.1.1 routes | inc 10.10

4. From the command output, it is established that the route has not been learned from A1.

5. Check for any filtering or route map present at the peer routers, which could cause nonentry of this route in the BGP table. Listing 11.19 shows the relevant part of the configuration that is checked using the command show running-config.

Listing 11.19 Output of show running-config Command at A1

router bgp 5555 neighbor 192.168.1.2 remote-as 6666 neighbor 192.168.1.2 distribute-list 1 in access-list 1 deny 10.10.1.0 access-list 1 permit any

Listing 11.20 shows the output of the command show running-config in B1.

Listing 11.20 Output of show running-config Command at B1

router bgp 6666 neighbor 192.168.1.2 remote-as 5555 neighbor 192.168.1.2 distribute-list 1 out access-list 1 deny 10.10.1.0 access-list 1 permit any

In Listings 11.19 and 11.20, access list 1 shown in bold may be present in either of the peer routers. This list is used for route filtering and can result in route 10.10.1.0/24 not being available in the BGP table of B1. You can remove or modify the access list as required to solve the problem.

1. Check the route advertisement at A1 if no routes are learned from B1. First, check the presence of the route in the BGP table of A1 by using the command:

   A1#show ip bgp 10.10.1.0 % Network not in table

   The command output shows that the route is not available in the BGP table. The advertisement needs to be checked. For this, execute show running-configuration and examine the relevant parts of the output. As 10.10.1.0/24 is not a classful network, the advertisement command should be in the form:

   router bgp 5555 network 10.10.1.0 mask 255.255.255.0

2. Check the availability of the route in the routing table if the network command is defined properly. This is performed using the command:

   A1#show ip route 10.10.1.0

Misconfiguration or unintentional error in configuration is a common reason for problems in BGP routing. In this section, we will discuss the common misconfiguration issues and ways to detect and correct them. Figure 11.6 shows the scenario illustrating a misconfiguration problem.

Figure 11.6: A BGP network showing misconfiguration problems between AS5555 and AS6666.

In Figure 11.6, misconfiguration can be caused because:

• Numbers may be incorrectly defined.

• Networks may not be declared.

Incorrectly Defined Numbers

In Figure 11.6, consider the configurations for A1 and B1. The configuration of A1 is:

router bgp 5555 neighbor 192.168.12.2 remote-as 5555 

The configuration for B1 is:

router bgp 6666 neighbor 192.168.12.1 remote-as 5555

In this case, the neighbor relationship will not be established between the routers. The corrected configuration for A1 is:

router bgp 5555 neighbor 192.168.12.2 remote-as 6666

The correct configuration for B1 is:

router bgp 6666 neighbor 192.168.12.1 remote-as 5555

Undeclared Networks

In Figure 11.6, let’s say the configuration of A1 is:

router bgp 5555 neighbor 192.168.12.2 remote-as 6666 network 192.168.2.0

The configuration of B1 is:

router bgp 6666 neighbor 192.168.12.2 remote-as 5555 network 192.168.2.0 network 192.168.16.0

In this case, not all the directly available networks in the routing table of A1 have been included in the BGP table. As a result, the networks that are not declared would not be reachable via BGP from any direct BGP peer or any other BGP-compatible router that is a part of the Internet. The corrected configuration for A1 is:

router bgp 5555 neighbor 192.168.12.2 remote-as 6666 network 192.168.2.0 network 192.168.16.0

The correct configuration for B1 is:

router bgp 6666 neighbor 192.168.12.2 remote-as 5555 network 192.168.2.0 network 192.168.16.0