From: jano@rhox.com.br
Date: Sun Jun 20 2004 - 18:07:56 GMT-3
Hi Ken,
Thank you for a great answer!
Regards,
Jano
> 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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