Make a RR advertise both paths for the same prefix toward its clients

[ThomasD86]

Hi, in the topology I am working with, I have 2 routers that are advertising a default route in a VRF. Those two routes are then advertised to a RR which then reflects them to its clients. My goal is to have the RR reflect both of the routes and then have the clients perform a routing decision on which path to utilize, rather than the RR.

On the RR this is what I am receiving:

Route Distinguisher: 210000:90
 *>i  0.0.0.0          172.16.0.49              0    100      0 ?
 *bi                   192.168.0.141            0    100      0 ?

This is the BGP configuration on the RR for the client that I want to receive both paths:

address-family vpnv4
  bgp additional-paths install
  neighbor 172.16.0.48 activate
  neighbor 172.16.0.48 send-community extended
  neighbor 172.16.0.48 route-reflector-client
  neighbor 172.16.0.48 advertise diverse-path backup mpath
maximum-paths 4

However with this configuration I am only able to change which of the paths gets advertised, by default it advertises the path that has .49 as next-hop, with the configuration above I can get it to switch to the .141:

Route Distinguisher: 210000:90
 *bia 0.0.0.0          192.168.0.141            0    100      0 ?

I tried to use this doc:
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/15-s/irg-15-s-book/irg_diverse_path.pdf
But all the configurations I've tried didn't change this behaviour, I am able to pick which of the two paths I advertise but, not both. Does anyone have any tip?

Thank you!

[David Ruess]

Hello,

Readong your document it looks like that’s diverse path only. Which means different then the best path and not more than one path as is BGPs default. You need BGP Multipath. In order for Multipath to kick in the routes must have the same path attributes until step 9 of the decision process. See this document and there is a link at step 9 for Multipath.

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

Hope that helps.

-David

[Harold Ritter · Accepted solution]

Hi @ThomasD86 ,

Please make sure that both PEs advertising the default route have a different route distinguisher (RD). RR will see the two paths as different (RD+ipv4 prefix) and will then not compare them to select a best path. The other PE should then receive the two paths.

Regards,

[ThomasD86]

Hi,

thanks for the help!

Reading the document I would have never gotten this to work without the suggestion from Harold as the two path have a different origin thus the RR will announce the one with the lowest RID to the clients. Configuring a different RD on both routers, worked just fine.

Thank you all and have a good day!

[Harold Ritter]

You are very welcome @ThomasD86 and thanks for the feedback