From: Yasser Aly (blackyeyes00@hotmail.com)
Date: Mon Mar 01 2004 - 19:49:02 GMT-3
Hi Packet Man,
The command " bgp neighbor .... allowsas-is " typical usage is with
networks having it's Core as
MPLS. In the real world, typically it is the service provider network that
is running MPLS.
In MPLS-VPNs, the service provider is interacting directly with the customer
on Layer 3 level, rather than just providing a layer 2 level. In this case,
the customer router other end will be the service provider PE (Provider
Edge) router. Other branches for this customer will be the same - terminated
on the Service Provider PE routers. Now to link the customer branches
together the customer will gain the benefit of any to any connectivity. In
this pattern you can not really say that there exist a hub and spokes where
a branch needs to pass by some hub to reach another branch.
Sorry for the introduction but this will help to explain your question
better.
No back to your question, if the customer is running BGP between him and the
service provider,
Once the routes arrive from a branch to the Service provider network it will
be tagged with the customer AS. Once the Service Provider pass it to its
desination - the other branch of the customer - it will be dropped if the
other branch was also running BGP with the SP using the same AS number.
Many solutions are offered to solve this problem, one of which is the
allow-as in command, inorder to fool BGP the other side and let the updates
pass to its destiantion.
If you ever thought of studying for the CCIE SP you will face lots and lots
of similar and other wierd scenarios that exist in the service provider
world.
HTH,
Yasser
>From: "Packet Man" <ccie2b@hotmail.com>
>Reply-To: "Packet Man" <ccie2b@hotmail.com>
>To: ccielab@groupstudy.com
>Subject: BGP neighbor....allowas-in
>Date: Mon, 01 Mar 2004 13:18:17 -0500
>
>Hi all,
>
>Trying to understand where, why and how to properly use the above command.
>The command reference says,
>
>"neighbor allowas-in
>To configure provider edge (PE) routers to allow readvertisement of all
>prefixes containing duplicate autonomous system numbers (ASNs), use the
>neighbor allowas-in command in router configuration mode. To disable the
>readvertisement of the ASN of the PE router, use the no form of this
>command.
>
>neighbor allowas-in number
>
>no neighbor allowas-in number"
>
>Could somebody explain what that's saying in plain English. And, maybe
>describe a scenario where using this command makes sense. I'm confused
>because the only that I know of where there would be deplicate ASN's is
>when ASn's have been prepended and I know this command isn't needed then.
>
>Also, is this command only applicable when MPLS is somehow involved in the
>config? Or, might this command be needed in the type of BGP scenarios
>possible in the lab?
>
>Thanks in advance
>
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