[c-nsp] OSPF VS ISIS

Oliver Boehmer (oboehmer) oboehmer at cisco.com
Mon Feb 27 06:43:51 EST 2006

Kshitiz Singhal <> wrote on Monday, February 27, 2006 12:17 PM:

> Dear All,
> We are coming up with new ISP and MPLS network and working to
> finalize IGP to be used, we are exploring two option OSPF and ISIS
> kindly help us on same and advise from your experience any issue like
> scalability, convergence etc with OSPF and ISIS
> and advise which IGP is better.

OSPF is as "good" as ISIS, your NOC's familiarity with either protocol
is most often the deciding factor (just as Robert pointed out in his

Both protocols scale very well, both have the same features (more or
less). When talking about the implementation in IOS, ISIS has some fast
convergence features (especially priority-driven RIB installation as
well as "advertise passive-only" to reduce the # of routes in your IGP)
which OSPF doesn't have, while OSPF currently supports LDP-IGP-sync
(ISIS will follow). But in a properly designed network, both protocols
can converge in sub-second..

Try to design your network so it can run in a single area/level which
makes things like TE/FRR or fast convergence easier later..


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