BGP Question

Unanswered Question
Mar 6th, 2007
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I apologize in advance if I can't word this question correctly.

In real-world scenarios, how is the "Next-Hop" address's network propagated to the internal routers. Let's say that there is a transit AS that is multihomed to two separate AS's: 10 and 20. AS 10 wants to send a packet destined for a network that 20 is advertising. When the packet is inside the transit domain, how does the "next-hop" address get in the routing table. For example:

B via

The network is external to the transit domain. Is a static route redistributed to the IGP considered good practice or is there a better method?

Please let me know if I need to clarify. Thanks!

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Jon Marshall Tue, 03/06/2007 - 02:45
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Hi Chris

If i understand correctly there are two ways of doing this.

For clarity lets call the transit AS 30.

1) You can do a redistribute connected on your EBGP router in AS 30 so that the link between AS 30 and AS 20 is propogated into the AS 30 domains IGP.

2) You can add an addtional line in the config on the EBGP peer in the transit AS that connects to AS 20 :

neighbor "IBGP peer in AS 30" next-hop-self.

This will mean that when the EBGP router in AS 30 that peers with AS 20 sends routes to IBGP peers within AS 30 it sets the next hop as itself and there should be a route to this in the IGP.

Generally speaking option 2 is the way to go in the real world.

Hope i have understood and this makes sense.



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