BGP best path selection understanding

[Vinayaka Raman]

CR 1 and 2 advertise prefix 10.66.0.0/20 Which one of these route is preferred by other peers ?

CR1#show ip bgp 10.66.0.0/20
BGP routing table entry for 10.66.0.0/20, version 941
Paths: (1 available, best #1, table Default-IP-Routing-Table, RIB-failure(17))
  Advertised to update-groups:
        1
  Local, (aggregated by 65457 10.66.0.253)
    0.0.0.0 from 0.0.0.0 (10.66.0.253)
      Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best
___________________________________________________________________

CR2#show ip bgp 10.66.0.0/20
BGP routing table entry for 10.66.0.0/20, version 944
Paths: (2 available, best #1, table Default-IP-Routing-Table)
  Advertised to update-groups:
        1
  Local, (aggregated by 65457 10.66.0.254)
    0.0.0.0 from 0.0.0.0 (10.66.0.254)
      Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best
  4809 4809, (aggregated by 65457 10.66.0.254)
    10.66.0.237 from 10.66.0.237 (59.43.0.45)
      Origin IGP, localpref 100, valid, external, atomic-aggregate

[Marwan ALshawi · Accepted solution]

This Is an aggregated bgp suumry route summarized on both routers and it looks like they send it out with same attributes

I would say other peers they could use the bgp nighbor ip as tie breaker here

If you want to make the first one preferred you can add/prepend more bgp AS numbers when you send the route through the scond one

HTH

[John Blakley · Accepted solution]

Vinayaka,

I believe that if you left it exactly as it is now, C1 would be preferred because of the lower RID, unless C2 is an older route. That selection criteria is way down on the list (under the oldest route which is another reason why I think C1 would be selected). I agree with marwanshawi that you should manipulate this with either using AS prepending or med (med will only be compared if these 2 routers are in the same AS). If you don't manipulate it, you're leaving BGP to do it's own guesswork as to what you want.

For example, if you created this route on C2 (10.66.0.254) and it propagated to your other routers, and then sourced the same route on C1 (10.66.0.253), the C2 router will be preferred for that route until the bgp sessions tear down. Then C1 will be preferred because it has the lowest RID. If you manipulate your routes outbound, you have more control over which one will always be preferred.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

HTH,

John