Problem in Propagating IBGP Route to IBGPEBGP Neighbor-Cause: IBGP Route Was Not Synchronized

Problem in Propagating IBGP Route to IBGP/EBGP Neighbor ‚ Cause: IBGP Route Was Not Synchronized

A scenario might arise in which an IBGP learned route is not propagated to any BGP neighbor, whether IBGP or EBGP. One case could be that when an IBGP-learned route is not synchronized, that route is not considered as a candidate to advertise to other BGP neighbors. As you remember from previous discussions in Chapter 14, a BGP route is synchronized only if it has been learned through an IGP or a static route first.

In Cisco IOS Software, BGP advertises only what it considers the best path to its neighbors. If an IBGP path is not synchronized, it is not included in the best path calculation.

Figure 15-17 shows the flowchart to follow to fix this problem.

Figure 15-17. Problem-Resolution Flowchart

Debugs and Verification

Refer back to Chapter 14 for details about the rules for synchronization.

Example 15-39 shows how an unsynchronized route would appear in BGP table.

Example 15-39 BGP Table with Unsynchronized Route

 R2#  show ip bgp 100.100.100.0  BGP routing table entry for 100.100.100.0/24, version 3 Paths: (1 available,  no best path  ) Flag: 0x208   Not advertised to any peer   (65201)     206.56.89.1 from 131.108.1.1 (1.1.1.1)  Origin IGP, metric 0, localpref 100, valid, internal,  not synchronized  

The highlighted output in Example 15-39 shows that R2 did not consider 100.100.100.0/24 as synchronized and failed to install it in the routing table; therefore, it did not advertise the route to any peer.

Solution

As discussed in Chapter 14, either turn off synchronization or make the routes synchronized by redistributing them in the IGP at the router that first introduced this route in IBGP domain. The following selection has an example to accomplish this.