Route table growth and hardware limits...talk to the filter

Sun Sep 9 04:13:52 UTC 2007

On Sat, 8 Sep 2007, Jon Lewis wrote:

> 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) 192.168.0.0/16   AS11111 AS22222 AS33333
> 2) 192.168.1.0/24   AS11111 AS22222 AS33333
> 3) 192.168.2.0/24   AS11111 AS55555 AS44444 AS33333
> 4) 192.168.3.0/24   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?

Based on what seems to be reported by the CIDR-REPORT, I would say that if 
#1 is revoked then it's likely all of the routes with the same AS Path 
will be revoked anyway.  But if not, rather than the router having to 
recalculate whether the more specifics should or should not be accepted 
at each routing update, you could apply the same principles that route 
flap dampening uses.  Reject paths #2 and #4 for X number of minutes 
before you bother checking again to see if the larger aggregate is still 
there.

> 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.  

Perhaps that's true, but perhaps another company like Juniper would 
implement it feeling that it would give their equipment an edge over their 
competitors.  If the number of routes was causing me a large problem with 
my routers, I would certainly look more closely at another vendor's gear 
if it offered a better solution for dealing with the problem than 
filtering based on RIR minimums. 

Forrest