RE: conditional default advertising in BGP with multiple routes

From: Joseph Brunner (joe@affirmedsystems.com)
Date: Wed Oct 17 2007 - 12:49:55 ART

No Darren, they must be in the bgp table to use the exist and non-exist
advertise maps...

There is a distinct difference.

You can

1. advertise a bgp prefix that was being suppressed if a bgp route leaves
the table (non-exist)
2. advertise a bgp prefix if one exists (exist map)

Or as the original poster was referring to advertise a default route with
conditions. The default-originate command can only match a route in the
routing table when deciding to advertise a default route to a peer. It can't
track a route in the bgp table.

So your options are "network 0.0.0.0" under the bgp process with what you
had below (and a static route to null0 for to get the default route into the
fib, to be matched by the bgp network command) or you can learn a default
route from a peer, and restrict where that goes with the advertise-map.

-Joe

-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Darren Johnson
Sent: Wednesday, October 17, 2007 4:00 AM
To: Koen Zeilstra; ccielab@groupstudy.com
Subject: Re: conditional default advertising in BGP with multiple routes to
monitor

Hey there. There is a much easier way using 'neighbor advertise-maps', check
it out here:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r
/rte_bgh2.htm#wp1071377

Yours would be something like:

router bgp x
neighbor x.x.x.x advertise-map DEFAULTROUTE exist-map MATCHINGROUTES

route-map DEFAULTROUTE permit
match ip address prefix-list thedeaultroute

route-map MATCHINGROUTES permit
match ip address prefix-list matchroutes

ip prefix-list thedefaultroute permit 0.0.0.0/0

ip prefix-list matchroutes permit 10.1.1.0/24
ip prefix-list matchroutes permit 192.168.4.0/24

This is saying that if 10.1.1.0 AND 192.168.4.0 are in the routing table,
advertise out the default route. This is known as BGP
conditional-advertisements....

Hope this helps

Dazzler

----- Original Message ----
From: Koen Zeilstra <koen@koenzeilstra.com>
To: ccielab@groupstudy.com
Sent: Tuesday, 16 October, 2007 10:13:33 AM
Subject: conditional default advertising in BGP with multiple routes to
monitor

Hi Group,

Please observe the following (I hope) interesting task.

Advertise a default route to a BGP neighbor conditionally. Watch two
routes in your routing table. When both routes are present the default
route should be announced to the neighbor.

I was thinking towards the following direction.

neighbor 10.0.0.1 default-originate route-map checkroutes
!
route-map checkroutes
  match ip address 10
  match ip address 20
!
access-list 10 permit 1.1.1.1
access-list 20 permit 2.2.2.2
!

However this route-map results in:

route-map checkroutes
  match ip addres 10 20
!

Which is an "OR" function afterall and not the "AND" we are looking for.

Any bright ideas on this particular assignment?

thanks in advance,

Koen

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