[c-nsp] TCAM refresher

[Justin M. Streiner]

Someone I know is considering upgrading the core network device in their 
data center to a 6500/Sup2/MSFC2.  They take three full BGP feeds from 
their transit providers.  My initial caution to them about going with the 
Sup2 instead of a Sup720/3BXL was that the Sup2 is limited to about 250k 
TCAM entries assuming they're not running uRPF.  Given the size of a full 
Internet routing table these days, that allows them a little bit of 
headroom, but not much, plus this doesn't take into account their existing 
access-lists, policies, local routes, multicast, Netflow, NAT, etc...
That said, it would seem that a Sup2 would work for now, but it won't be 
very long before it would need to be upgraded, i.e. they'd be better off 
going with the 720/3BXL straight out of the gate.

IIRC, if the TCAM table fills up, certain operations will either 1. fail, 
or 2. be handled in software, with the associated performance penalties.
Am I correct in saying this?

Also, how much TCAM headroom should be allowed for things like significant 
routing topology changes, i.e. adding a new BGP neighbor, general routing 
table churn, etc?

Thanks
jms