Redundancy & Summarization

Jack Bates jbates at brightok.net
Fri Aug 21 12:32:18 CDT 2009


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




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