BGP path selection in route reflector

[suryakant.chavan]

Hi Expert,

If RR is receiving same prefix (prefix A) from two routers (Router B and C) with same AS path then which route  (with path B or c) will get advertised to route reflector client.

I thought that RR should propagate most stable route to RR client but it was propagating route with best router id.

Can someone help me to understand..

[Georg Pauwen]

Hello,

there is nothing in the BGP best path algorithm that specifies a 'stable' path...how do you define a stable vs an unstable path ?

[Richard Burts]

The original question seems to imply that the path selection for RR is different from regular BGP path selection. As far as I know there is not anything different in path selection for RR. See this link for some details about path selection.

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

HTH

Rick

[suryakant.chavan]

Hi Richard,

I observing in my environment that we are learning route 10.10.10.0/24 on RR from router 1 (Router id 192.168.10.1 ) and router 2 (router id 192.168.10.2 ) with same AS path. 

RR is propagating route received from router 1 due to lowest router id but if if due to some reason if router 1 is not receiving (from neighbor) route and sending to RR  , the RR is refreshing the route in RR client i.e route from Router 2. But when router 1 again learn the roue from neighbor and advertised to RR , RR send Router 1 route to RR client.  

My question is if route learning from Router 2 in RR routing table for more time period then why RR send R1 router route to RR client 

[Sebastian Fernandez]

This is correct/expected behavior, BGP runs a scanner process that checks for next-hop and best-routes.  As soon as a better route is received, RR will announce it further to RR-clients. This has nothing to do with the age of the route in RR routing table. RR will only announce the best BGP path to RR-clients.  Since R1 is provides RR a better route with lowest router ID, it marks it as the best path and then propagates it to other RR-clients.

Hope this all make sense now. 

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Sebastian