Re: BGP and AS loop prevention

I would re-check your setup.

You are right, the allow-as will simply allow in the local AS x number of
times in the path. This is done on the receiving end.

If you changed your set up some, and one side has the AS 101, is the problem
present? I might suggest to try this and make sure all BGP is cool first.

Also, you can prepend an AS on advertisement, so you can leave the one side
with AS 101 as is and simply send out a route with AS 100 in the path. The
AS 100 router should not accept it. This is another way to test the
allowed-as command.

Create a new loopback on AS 101 and prepend AS 100 in it.

HTH,

Andrew Lee Lissitz

On Mon, Oct 19, 2009 at 11:12 PM, Nate Lee <natetlee_at_gmail.com> wrote:

> I have a scenario labbed up where I have an discontiguous BGP AS split by
> another AS, something like this: (AS100) <> (AS 200) <> (AS 100).
>
> I am trying to get the prefixes on the AS 100 routers to show up in each
> others BGP tables. I have implemented the neighbor x.x.x.x allowas-in
> command on the AS 100 routers on their neighbor statements facing the AS
> 200
> routers but it does not seem to work. When I do a SHOW IP BGP NEIGH
> x.x.x.x
> ADVERTISED-ROUTES on the AS 200 routers, I see that the AS 200 router will
> not send the route that originated in the far side AS 100.
>
> Is there some sort of configuration needed on the AS 200 routers to allow
> them to send a route with an AS PATH that contains the AS they are sending
> into? I was under the impression that they do not perform an outgoing AS
> PATH check.
>
> Thanks!
>
> Nate
>
>
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-- 
Andrew Lee Lissitz
all.from.nj_at_gmail.com
Blogs and organic groups at http://www.ccie.net