The MPLS-OPS Archive

[Rajiv Asati <rajiva@cisco.com>]

Ran,

>My question is - is it possible to configure this? when I want BGP to
>carry the next-hop of the other AS?

Yes. It should work.

 From your text, it seems as if you are trying to do Inter-AS Option C. 
(However, the same is applicable to Option B as well).

What you need is the ability to set up BGP+Label session. With rfc3107 
(send-label) being configured such that AS2_PE receives the route to AS1_PE 
via BGP+Label session, then AS2PE would have the following info -

AS1_PE - BGP route, next-hop= AS2_ASBR, Label1
AS_ASBR - IGP route, next-hop=AS2_P, label2

In other words, AS2_PE would have to build two label stacks to get AS1_PE.
Then, add the 3rd label for the VPN prefixes (if any).

Keep in mind that the 3 label stack will be only within the ASes in each 
direction of the traffic.

Cheers,
Rajiv

At 03:08 PM 2/14/2005, Ran Liebermann wrote:
>Greetings,
>
>I've just encountered a problem, for which I found a workaround, but I
>still would like to configure it as I wanted in the first place.
>
>Please see the following diagram:
>
>CE --- AS1_PE --- AS1_P --- AS1_ASBR --- AS2_ASBR --- AS2_P --- AS2_PE --- CE
>
>Let's say I'm running AS2, and everything currently works well.
>Now I want to configure the IGP of my AS to carry only the loopbacks
>of my network (and ofcourse also the connected interfaces on which the
>IGP is running), and the iBGP to carry all the redistributions of
>static and connected routes.
>When I do that, the /32 route of the next-hop in AS1 becomes a BGP
>recursive route, pointing to the loopback of AS2_ASBR, instead of
>pointing to the other side of the LSP that instructed the packets to
>be carried out of my ASBR to AS1_ASBR.
>
>When this recursive route happens, then if I try, for example, to do
>"show mpls for vrf X <prefix>" then I see the outgoing label is the
>same as the loopback of AS2_ASBR.
>When the packet arrives at AS2_ASBR (without the outer label) then the
>inner label is acted upon, and although the label is identical to the
>label that AS1_ASBR is waiting for, AS2_ASBR doesn't know this label
>(or has some other prefix for it) and carries it to a different place.
>
>My question is - is it possible to configure this? when I want BGP to
>carry the next-hop of the other AS?
>
>Thanks,
>--
>Ran.
>
>P.S.: The workaround was to configure the interfaces towards AS1 to be
>carried in the IGP.
>
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