Re: BGP "Allowas-in" -- scenarios to use this?

On Tue, Aug 18, 2009 at 14:34, Adrian<ccie2323_at_gmail.com> wrote:
> dun think it will ever happen. unless for private AS. B Have 2 remote sites
> that is having the same as that transit a different AS in the middle.
>
> AS100-AS200-AS100

This is actually somewhat common in MPLS VPN deployment scenarios.
Imagine Enterprise (say with AS65000) that has MPLS provider with AS
65100. They have several sites and run BGP:

(site1: AS65000) --- (MPLS: AS65100) --- (site2: AS65000)
(site3: AS65000) ----^

As you can see, all customer sites are in AS65000. When update from
site1 arrives to either site2 or site3, AS_PATH will look like "65001
65000". Site2 will, of course, reject this as looped.

One of the solutions how to overcome this situation is to use "bgp
allowas-in". There are others, but they require configuration by
service provider - who may be unwilling to change their configurations
and let customers deal with their issues, without getting too
involved.

--
Marko
CCIE #18427 (SP)
My network blog: http://cisco.markom.info/
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