Re: BGP and AS loop prevention

Hey team,

Johnny, I might be mistaken, but I gather he has already tried to allow BGP
to accept the route and the routes are not being propagated.

A quick lab test: I just set up a lab with two routers; took about 2 minutes
to complete.

R1 <---> R2

R1 is in AS1
R2 is in AS2

Created loops on each and each router sees the routes ok.

I created a new loop on R1 and R2 receives this new route no problem.

I create a route-map on R1 and prepend this newly created route with AS2 in
the as-path.

Problem ...

Problem resolved on R2 with allow-as.

This was a simple test, and if anyone out there wants to learn / practice
this, it only takes two routers. Pretty quick and easy, but helps to show
the problem / solution.

HTH,

Andrew

On Mon, Oct 19, 2009 at 11:46 PM, Johnny B CCIE <jbccie_at_gmail.com> wrote:

> Nate,
>
> Do you think there is a reason that the AS100 will not accept another
> AS100 sending it prefixes? Think about for a moment. What does a BGP
> Speaker do with prefixes if it thinks they came from somewhere else?
> Now ask yourself can you change this behavior and if you can then how
> will you do it?
>
> 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