North American Network Operators Group
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RE: BGP Filtering

  • From: Jon Lewis
  • Date: Tue Jan 15 14:05:46 2008

On Tue, 15 Jan 2008, Ben Butler wrote:

I want a filter that will automatically match the shorter prefixes that
match any longer prefix, once I can match them I can drop them.
I don't want to manually configure a static prefix list for lots and
lots and lots of reasons.
If the longer prefix disappears from the route table I want to stop
filtering the shorter prefixes - automatically.


This was talked about / requested several months ago on cisco-nsp.  IIRC,
the thread ended along the lines of don't hold your breath.
Implementation of this sort of feature is very icky (lots of details you
may not be considering) and why should cisco spend time writing this code
when they can sell you a bigger router instead?

If the filter has to remember routes that are filtered so they can be automatically unfiltered if their covering prefix is withdrawn, then where's your savings? You can't have tea and no tea simultaneously. You want to filter routes, but keep them around (and extra pointers connecting their covering prefixes to them) in case they're needed in the future...sort of like partial soft-reconfig. On a platform like the 6500 where you may have surplus RAM but limited TCAM, that could work...on the software routers where RAM is the limiting factor it's not going to help.

--
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 Jon Lewis                   |  I route
 Senior Network Engineer     |  therefore you are
 Atlantic Net                |
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