[c-nsp] BGP question : What's the best way for filtering outgoing prefixes?

[Jay Nakamura]

While testing, I am wondering, is it standard practice to clear my
community strings from routes before going to peer/transit?



On Thu, Aug 18, 2011 at 4:00 PM, Jay Nakamura <zeusdadog at gmail.com> wrote:
> This is a bit complicated.  Let's say we are provider X.  X is
> connected to transit provider A and B.  X currently uses prefix-list
> to filter outgoing BGP announcement.
>
> We are now getting a customer that wants to multi-home, so their
> transit provider is X and C.  We gave them a /24 from our block, let's
> call it IP1.
>
> I was simulating how I should configure our routers so it was secure
> and did all the right things when I noticed IP1 route coming in from
> provider A is getting advertised to provider B through us.  It makes
> sense since it passes our outgoing prefix list.  (So, AS path was
> "AS_X AS_A AS_Customer" into provider B)
>
> What's the best way to prevent this?  Here are the two options I was
> thinking of doing
>
> Option 1
> Set all routes learned from A and B with unique community, and filter
> out any routes with that community for outgoing routes to A and B.
>
> Option 2
> Filter on AS-Path for routes going out A and B with
> <AS-X>$
> <AS-X>_(<AS_CUSTOMER>)+_$
> (I think, I haven't looked closely at AS path syntax)
>
> With Option 1, I don't have to do anything when we add another BGP
> customer but not sure what the overhead of tagging all routes coming
> in with community is.  With Option 2, I have to edit the AS-path every
> time we add a customer.
>
> Is there a better option?
>