what is the winner between bgp aggregate route and bgp updated route.

[syjeon]

Hi,

we are experiencing a trouble about BGP. but, I found that the router havnig a issue is aggregate the network as following.

router bgp x.x.x.x

aggregate-address x.x.x.x 255.255.128.0 summary-only

!

routerA#show ip bgp x.x.x.x

BGP routing table entry for x.x.0.0/17, version 20652067

Paths: (2 available, best #1, table Default-IP-Routing-Table)

  Advertised to update-groups:

     1          2        

  Local, (aggregated by xxx x.x.x.x)

    0.0.0.0 from 0.0.0.0 (x.x.x.x)

      Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best

  Local, (aggregated by xxx x.x.x.x), (received & used)

   x.x.x.x(metric 25630720) from x.x.x.x (x.x.x.x)

      Origin IGP, metric 0, localpref 200, valid, internal, atomic-aggregate

      Originator: x.x.x.x, Cluster list: x.x.x.x

There are 2 avilable paths about rotue x.x.x.x but, due to aggregation from the router is preferring to local aggregate route. but, they have to pick up bgp update route. not local aggregate route.

My point of questions is local aggregate route is considered best path than bgp updated route ?

[Peter Paluch]

Hello syjeon,

My point of questions is local aggregate route is considered best path than bgp updated route ?

The aggregate route is a locally originated route and Cisco BGP implementation assigns the weight of 32768 to all locally originated routes. As the weight is the first attribute evaluated in Cisco's BGP best path algorithm, it makes the locally generated aggregate route to be preferred as the received BGP route has a weight of 0 (in fact, it has no weight because this attribute is local to a router, and received routes are internally assigned the default weight of 0). Note that your own output says that the weight of the locally originated route (having the next hop 0.0.0.0) is 32768.

You can modify this behavior by setting the weight of the locally originated aggregate to 0 using a proper route-map, for example:

route-map NoWeight permit 10

  set weight 0

router bgp N

  aggregate-address x.x.x.x 255.255.128.0 route-map NoWeight summary-only

As received routes have no weight (it is a local attribute only), their default weight for internal comparisons is also set to 0. By setting your own locally injected route's weight to 0, you are making the weight values identical, forcing the BGP best path algorithm to move to the next evaluated attribute - the local preference.

Best regards,

Peter

[Raju Sekharan]

Hi

The weight of the local-aggregate by default is 32768. So that is best path.

Why are you summarizing the same subnet at multiple routers? When you summarize a route, the router summarizing will have child routes and the summarized subnet pointing to null. So any IP matching the child routes will be forwarded using that routes and the rest of the IPs in that supernet will get dropped by the summarized subnet pointing to null

Thanks

Raju

[Peter Paluch]

Raju,

Why are you summarizing the same subnet at multiple routers?

Well, isn't that common in multihomed ASes where AS border routers summarize the AS towards ISPs?

Best regards,

Peter

[Raju Sekharan]

Hi Peter,

I missed that. Thank you for pointing in out

Thanks

Raju