[116844] in North American Network Operators' Group
Re: Redundancy & Summarization
daemon@ATHENA.MIT.EDU (Jack Bates)
Fri Aug 21 13:41:19 2009
Date: Fri, 21 Aug 2009 12:32:18 -0500
From: Jack Bates <jbates@brightok.net>
To: Grzegorz Janoszka <Grzegorz@Janoszka.pl>
In-Reply-To: <4A8ECBE5.5090709@Janoszka.pl>
Cc: nanog@nanog.org, "Gaynor, Jonathan" <Jonathan.Gaynor@fccc.edu>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
Grzegorz Janoszka wrote:
>
> No, BGP does not work this way. But you may force some carriers to have
> only /16. First, you may try to announce the /17's with the community
> no-export, so they will be seen only by your direct ISP, not by the rest
> of the world. Or you may try to use some other communities to limit
> announcements of your shorter prefixes, only to some part of the world.
>
Actually, BGP does work that way. The goal is for both /17's to normally
make the route decisions, but if one of them disappears, there is a
covering /16 route. While this normally wouldn't be a problem, there are
places that have issues with their routing table size and do strange
modifications to which prefixes they accept.
I'd be more concerning if it was a bunch of /24's in a /16 cover, but
given the extent of only having 3 prefixes, MOST policies would accept
all 3 just fine.
That being said, there is still the possibility of some traffic coming
the wrong way, but it should be very small (less than if you sent both
/17's out both providers and prepended).
Jack
