BGP advertising route-map issue

jvulto · ‎07-13-2007

Imagine a situation in which a EBGP session is setup between two routers A and B.

Router A advertises routes that are configured using the 'redistribute static' statement AND defined in an ACL that is called upon using a route-map.

In this situation I want to add a new static route to network X at router A, pointing somewhere to my inside networks.

Then also I want to advertise this network X via EBGP to router B.

If I follow the following procedure, network X will NOT be advertised to B unless clearing the BGP process:

*. redistribute static (in place already) using route-map;

1. add the static route to network X;

2. add the network X prefix into the ACL.

If however I follow the following procedure, network X WILL be advertised to B WITHOUT clearing the BGP process:

*. redistribute static (in place already) using route-map;

1. add the network X prefix into the ACL.

2. add the static route to network X;

Can somebody explain the difference in behaviour?

bjornarsb · ‎07-13-2007

Hi,

I just remeber from the cource that this is how i works. Manipulating BGP inbound and outbound parameters reuires a soft clear of the bgp session.

BR,

Bjornarsb

Edison Ortiz · ‎07-13-2007

That's a very interesting problem and I will have to lab it out but I think it has to do with the RIB refresh on the 2nd option somewhat kicking BGP to update/refresh its table as well.

On the first option, adding the static route refreshes/updates RIB but the network is not yet allowed by the route-map while on the second option, when the RIB updates/refreshes, the route-map is allowing that network already.

Pretty cool trick.

Pavel Bykov · ‎07-13-2007

That is cool. I agree with Edison.

Adding a static route triggers local RIB recalculation. Check out various debugs.

For example ip routing debug and some BGP debugs:

2851_hra_lab(config)#ip route 0.0.0.0 0.0.0.0 10.96.24.2

2851_hra_lab(config)#

Jul 14 09:38:33.707: RT: closer admin distance for 0.0.0.0, flushing 1 routes

Jul 14 09:38:33.707: RT: NET-RED 0.0.0.0/0

Jul 14 09:38:33.707: RT: SET_LAST_RDB for 0.0.0.0/0

NEW rdb: via 10.96.24.2

Jul 14 09:38:33.707: RT: add 0.0.0.0/0 via 10.96.24.2, static metric [1/0]

Jul 14 09:38:33.707: RT: NET-RED 0.0.0.0/0

Jul 14 09:38:33.707: RT: default path is now 0.0.0.0 via 10.96.24.2

Jul 14 09:38:33.707: RT: new default network 0.0.0.0

Jul 14 09:38:33.711: RT: NET-RED 0.0.0.0/0

Jul 14 09:38:33.711: BGP- ATF: EVENT 0.0.0.0/0 RIB update DOWN

2851_hra_lab(config)#

Jul 14 09:38:33.711: BGP- ATF: EVENT 0.0.0.0/0 RIB update UP

2851_hra_lab(config)#

Jul 14 09:38:38.708: RT: NET-RED 0.0.0.0/0

Jul 14 09:38:38.708: BGP- ATF: EVENT 0.0.0.0/0 RIB update UP