Route table growth and hardware to the filter

Jon Lewis jlewis at
Sun Sep 9 02:07:18 UTC 2007

On Sun, 9 Sep 2007, Randy Bush wrote:

>> Maybe this is a dumb question, but why isn't there a BGP option to just
>> filter more specific routes that have the same AS path as the larger
>> aggregate?
> i think i filed that request case three or more years ago.  zero response.

IIRC, this has come up on cisco-nsp before, and the response has been that 
it's very "icky" to do and doesn't really save anything on most platforms.

In the example case of

1)   AS11111 AS22222 AS33333
2)   AS11111 AS22222 AS33333
3)   AS11111 AS55555 AS44444 AS33333
4)   AS11111 AS22222 AS33333

Forrest says the router should be smart and reject paths 2 and 4 because 
they're covered by 1.  Now what happens when 1 is revoked?  Do we lose 
connectivity to 2 and 4, or does the router have to keep track of all 
these dependant routes and reinstall 2 and 4 when 1 is lost?

Granted, the overhead involved would maybe be worth it on a platform like 
the 6500/7600, where you can end up with a surplus of RAM and not enough 
TCAM, if only the "active" routes were stored in TCAM, but it's not 
exactly in cisco's best interest to extend the life of gear they'd like to 
see replaced with new cisco gear.  I just can't understand why they 
won't/haven't done a Sup32-3bxl for those using this platform but not 
moving enough Gbps to need the traffic capabilities of the Sup720-3bxl.

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