BGP: Why does Cisco router sometimes advertise routes to the peer that it learned the route from?

[jesses5]

I'm testing Cisco Cloud Services Router in a virtualized environment. See the attached image for the topology I've setup. Basically I have 3 CSR's and one vShield Edge that are talking eBGP to each-other.

The problem I'm having is that sometimes, the Cisco CSR will advertise a route back to the peer it learned the route from.

CSR(T) advertises 192.168.1.0/24, and Edge(S) advertises 192.168.0.0/24, so that Source and Target VM can communicate. CSR (1) and (2) sit between CSR(T) and Edge(S) and re-advertise the routes learned.

If CSR(T) is peered ONLY with one of the middle CSR's (1 or 2, but not both), then everything is fine. When I have it peered to both at the same time, then CSR(T) advertises 192.168.0.0/24 back to both CSR(1) and CSR(2). 

CSR(1) is doing something similar - advertising 192.168.1.0/24 to CSR(T) and advertising 192.168.0.0/24 to Edge(S). CSR(2) is configured nearly identically but isn't doing that.

I'm just wondering under what circumstances will a Cisco BGP speaker advertise a route back to the peer it learned it from. It seems like it should never do that. Is there a bug in the Cisco BGP code that I'm running into?

[milan.kulik]

Hi, this question has been discussed here several times in the past https://supportforums.cisco.com/discussion/10875846/bgp-behaviour#3052977 e.g. And the conclusion was: No RFC says: "BGP speaker should not advertise a route back to the peer it learned it from." Cisco routers sometimes don't advertise back and sometimes do. IMHO, it's a question for IOS programmers. Best regards, Milan