Hi all experts.
I have a simple topology.
Over here, All 3 routers are peering with each other. R1 is advertising a prefixm 10.1.1.0/24 to R2 and R3. R3 is receieving the same prefix from R2, When i issue show ip bgp neighbor 126.96.36.199 advertised-routes on R3, i can see the prefix 10.1.1.0/24 being advertised back to R1, but on R1 when i issue this command, show ip bgp neighbor 188.8.131.52 received-routes, i dont see any result. kindly note, i have issued soft-reconfig inbound from both neighbors.
Does this mean, that R1 will not at all keep the copy of this prefix, since its totally invalid ? can some one refer me a doc that describes this behaviour ?
In addition to what KFK noted, have a look at this document for more explanation of why routes are being ignored:
Assume that all paths that a router receives for a particular prefix are arranged in a list. The list is similar to the output of the show ip bgp longer-prefixes command. In this case, some paths are not considered as candidates for the best path. Such paths typically do not have the valid flag in the output of the show ip bgp longer-prefixes command. Routers ignore paths in these circumstances:
Paths that are marked as not synchronized in the show ip bgp longer-prefixes output
If BGP synchronization is enabled, there must be a match for the prefix in the IP routing table in order for an internal BGP (iBGP) path to be considered a valid path. BGP synchronization is enabled by default in Cisco IOS® Software. If the matching route is learned from an Open Shortest Path First (OSPF) neighbor, its OSPF router ID must match the BGP router ID of the iBGP neighbor. Most users prefer to disable synchronization with use of the no synchronizationBGP subcommand.
Note: Synchronization is disabled by default in Cisco IOS Software Release 12.2(8)T and later.
Paths for which the NEXT_HOP is inaccessible
Be sure that there is an Interior Gateway Protocol (IGP) route to the NEXT_HOP that is associated with the path.
Paths from an external BGP (eBGP) neighbor if the local autonomous system (AS) appears in the AS_PATH
Such paths are denied upon ingress into the router and are not even installed in the BGP Routing Information Base (RIB). The same applies to any path that is denied by a routing policy that is implemented via access, prefix, AS_PATH, or community lists, unless you have configured soft-reconfiguration inbound for the neighbor.
If you enabled bgp enforce-first-as and the UPDATE does not contain the AS of the neighbor as the first AS number in the AS_SEQUENCE
In this case, the router sends a notification and closes the session.
Paths that are marked as (received-only) in the show ip bgp longer-prefixes output
The policy has rejected these paths. However, the router has stored the paths because you have configured soft-reconfiguration inbound for the neighbor that sends the path.
Using soft reconfiguration inbound merely instructs the router to store ALL the prefixes from a specific neighbor in a separate memory buffer. These prefixes are then handled by inbound prefix filtering before they make their way to Adj-RIB-in and finally to the BGP table. So, soft reconfiguration has nothing to do with your problem.
Hope this made it a bit clearer.