Advertised BGP Networks using Static Routes

Unanswered Question
Feb 24th, 2009

If a static route is configured to advertise a /24 network using a next hop address that is behind an interface on this router that is currently down, will those networks be advertised out via BGP.

Please advise and thank you!

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Yudong Wu Tue, 02/24/2009 - 11:19

should not since that static route will not be in routing table any more.

Mohamed Sobair Tue, 02/24/2009 - 11:36

Hi,

If the next-hop is not reachable, then no rules applied.

BGP first check the next-hop reachability by performing lookup in the Ip routing table (RIP) , if the next-hop not reachable, then No network will be advertised.

HTH

Mohamed

Giuseppe Larosa Tue, 02/24/2009 - 12:12

Hello Matt,

the BGP process executes routinely every x seconds a control of BGP next hops reachability so at the next control if the next hop is found unreachable all BGP prefixes that have this BGP next-hop are removed from the BGP table and a withdrawn message is prepared and sent out to all BGP neighbors telling to remove those prefixes.

Edit:

the check of BGP next-hop is made by examining the IP routing table not by sending probe packets.

Hope to help

Giuseppe

glouis Thu, 02/25/2010 - 09:08

As Matt stated,

     Edit:

     the check of BGP next-hop is made by examining the IP routing table not by sending probe packets.

BGP will advertise any network (specified by network statement)  if it is known via an IGP or in the IGP routing table.

So,if you do a "show ip route" and see a route listed (static or otherwise) that matches a BGP network statement, BGP will advertise the route.

Use the sho ip bgp neighbors X.X.X.X advertised-routes  command to see whats being sent to neighbot. Where x is neighbor ip that has adjacency up.

It is easy enough to test in a lab in 5 mins. using GNS3 or something like that.

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