Redundancy & Summarization

Grzegorz Janoszka Grzegorz at
Fri Aug 21 11:31:33 CDT 2009

Gaynor, Jonathan wrote:
> My institution has a single /16 spread across 2 sites: the lower /17 is
> used at site A, the upper /17 at site B.  Sites A & B are connected
> internally.  Currently both sites have their own ISPs and only advertise
> their own /17's.  For redundancy we proposed that each site advertise
> both their own /17 and the whole /16, so that an ISP failure at either
> site would trigger traffic from both /17s to reconverge towards the
> unaffected location.
> My worry/question: will carriers down the line auto-summarize my
> advertisements into a single /16, resulting in a 'load sharing' while
> both sites are active?  If you're a backbone carrier and you saw x.x/16
> and x.x/17 (or x.x/16 and x.x.128/17) being advertised from the same
> peer would you drop the longer match?

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.

Grzegorz Janoszka

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