From: Jonathan Hays (nomad@gfoyle.org)
Date: Mon Mar 01 2004 - 15:38:09 GMT-3
you wrote:
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
>Behalf Of Packet Man
>Sent: Monday, March 01, 2004 1:18 PM
>To: ccielab@groupstudy.com
>Subject: BGP neighbor....allowas-in
>
>
>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
= = =
Cisco's wording is somewhat convoluted but what the documentation is
referring to is the situation where Router X is receiving BGP
advertisements containing its own AS number. Normally this condition
would indicate a loop and the update would be dropped. But it might be
the case that another AS exists external to Router X's AS that has the
same AS number and you want Router X to receive those updates. I can't
say I've seen such a situation in the real world but I did come across
it in a practice lab last year.
HTH,
Jonathan
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