[c-nsp] faster IGP convergence

[Oliver Boehmer (oboehmer)]

> "how faster" is IS-IS compared to OSPF? We are now seeing the limits
> of OSPF since we are running voice over an originally Internet-only IP
> network, and I am wondering if moving to IS-IS would be worth it...

Generally speaking, ISIS and OSPF can converge equally fast (sub-second
for 2000 IGP prefixes) if tuned and designed appropriately. All relevant
enhancements are in 12.0(27)S and later releases. Key factors include
rapid failure detection (i.e. POS/carrier-delay) and exponential backoff
for SPF (i.e. fast initial reaction, but backoff to avoid churns). 

There is currently only one feature ("IS-IS Support for Priority-Driven
IP Prefix RIB Installation") which plays a role in some deployments
which is not available for OSPF.

	oli