BGP path selection in RR environment strange behavior

[Martin12]

Hi Guys,

I encountered something strange that I cannot explain to myself ...

The bestpath selection should have gotten to the point where everything is a tie up til router-ID.

Then lowest router-ID should be selected, right? Check for yourself --

I thought that it has something to do with the cluster list but it turned out that since the BGP router ID is fitting inside the /24 prefix that was in question, the router made decision that higher RID or not, it would go that way anyway.

Can someone explain why at this point the BGP algorithm did something unexpected ? I think there is some loophole that is testing the forwarding towards the RID and it matched the prefix itself (even though originator is another device) and it simply overrode the lowest BGP RID rule ... When I changed it to something else higher than 150.1.5.5 and not within 222.22.2.0/24 it worked just fine

BGP routing table entry for 222.22.2.0/24, version 127

BGP Bestpath: compare-routerid

Paths: (2 available, best #1, table default)

  Advertised to update-groups:

     4          5          6        

  Refresh Epoch 8

  254, (Received from a RR-client)

    192.10.1.254 (metric 2560000512) from 155.1.23.2 (222.2.2.2)

      Origin incomplete, metric 0, localpref 100, valid, internal, best

      rx pathid: 0, tx pathid: 0x0

  Refresh Epoch 7

  254

    192.10.1.254 (metric 2560000512) from 155.1.0.5 (150.1.5.5)

      Origin incomplete, metric 0, localpref 100, valid, internal

      Originator: 222.2.2.2, Cluster list: 150.1.5.5

       rx pathid: 0, tx pathid: 0

[jalejand]

It must be because of the Originator 222.2.2.2:

BGP Best Path Selection:

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

The router ID is the highest IP address on the router, with preference given to loopback addresses. Also, you can use the bgp router-id command to manually set the router ID.

Note: If a path contains route reflector (RR) attributes, the originator ID is substituted for the router ID in the path selection process.

• If the originator or router ID is the same for multiple paths, prefer the path with the minimum cluster list length.

  This is only present in BGP RR environments. It allows clients to peer with RRs or clients in other clusters. In this scenario, the client must be aware of the RR-specific BGP attribute.