Redundancy & Summarization

Gaynor, Jonathan Jonathan.Gaynor at
Fri Aug 21 10:57:49 CDT 2009

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?

Regards and thanks,

Jon Gaynor, Senior Network Engineer
Fox Chase Cancer Center
(215) 214-4267, jonathan.gaynor at

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