From: Kenneth Wygand (KWygand@customonline.com)
Date: Sun Jun 20 2004 - 15:41:40 GMT-3
Jano,
The reason you are having this problem is because of the underlying purpose of BGP Synchronization (which you mentioned you disabled). BGP synchronization would remove the 10.0.0.0 route from Neighbor B's routing table because he doesn't know how to get to 10.0.0.0 through an IGP. If he -did- know how to get to 10.0.0.0 through an IGP, you wouldn't have the problem (most likely) because the External router would also know how to reach 10.0.0.0 which is your destination network.
By disabling synchronization, you are telling Neighbor B, "put 10.0.0.0 in your routing table even though you don't know how to get there via an IGP - let's hope that the network is actually reachable!".
Synchronization can -safely- be disabled in two cases. The first case is where all transit routers in your AS (15) run IBGP (which you stated is -not- the case). If this were the case, your External router would know how to get to 10.0.0.0 through its own BGP route.
The other time synchronization can safely be disabled is if your AS is -not- a transit AS for any other AS's. I'm not 100% sure, but I think this assumes that if your AS is not a transit AS, then your AS is a stub AS and should have default routes pointing out towards the Internet (or other core routers). If I am incorrect in this assumption, hopefully someone will correct me.
Now on to how to fix this - you have a few options. First, you can redistribute BGP into the IGP routing protocol run by your external router so he learns how to get to 10.0.0.0. Secondly, you could add the "BGP next-hop-self" command to your neighbor configuration on Neighbor A pointing to Neighbor B. This will tell Neighbor B to send packets to Neighbor A instead of directly to the router in AS 2 (which is default behavior for BGP). Thirdly, you could also run BGP on your External router. Fourthly, you could establish a GRE tunnel through your external router and run BGP over the tunnel.
I'm sure there are a lot more ways to accomplish this as well. If this was a practice lab with the exact requirements you provided, I would opt for the BGP next-hop-self command.
Hope this helps!
Ken
-----Original Message-----
From: nobody@groupstudy.com on behalf of jano@rhox.com.br
Sent: Sun 6/20/2004 10:52 AM
To: ccielab@groupstudy.com
Cc:
Subject: iBGP and external router
Hi,
I have iBGP (AS 15) running between two routers, but they are not directly
connected, but connected through an external router:
NeighborA ------ ExternalRouter ------- NeighborB
<------------------------------------------>
iBGP AS 15
[AS 2] ------ [AS 15]
eBGP
Through eBGP AS 2 advertises a route (lets say 10.0.0.0) to AS 15
(NeighborA).
NeighborA advertises this route to NeighborB (no synchronization).
The problem:
When NeighborB tries to ping 10.0.0.1, it sends the packet to the nexthop,
ExternalRouter. But ExternalRouter does not have a route to network
10.0.0.0 and drops the packet (because it doesn't participate in BGP).
The question:
How do I fix this?
Regards,
Jano
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