BGP Next hop behaviour

[mahmoodmkl]

Hi,

Please i need to clarify how the bgp next hop will behave in the below topology.

Imagine that i have 5 Routers R1,R2,R3,R4,R5 and R4 and R5 are provider routers.R2 is peering with R4 with EBGP and with R1 IBGP.

R4 is peering with R5 EBGP and R1 with IBGP.R1 is acting as route reflector for R2 and R3.

So when R2 recieves any update from R4 it will advertise to R1 without changing the next hop which will be the link between R2 and R4 and the same applies to R3 and R5 and i assume this is the correct process but i am seeing different output that the routes which are advertised from R2 to R1 and reflected to R3 are having the next hop of R2 IBGP not the EBGP link.

Please clarify whether this is the expected behaviour.

Thanks

[Jon Marshall]

The next hop should not be changed unless you have used the "neighbor <peer IP> next-hop-self" command.

If you haven't then R1 will see the next hop as R4 or it should do.

Which usually means it can't advertise it because it does not know how to reach the next hop IP.

If this is not the case and you have not used that command can you do a quick schematic with IP address details and pick a route being advertised by R4 and do a "sh ip bgp <route" from all routers and post back.

Jon