I asked on comp.dcom.sys.cisco a few days ago about the advantages of
IS-IS over OSPF. Dave Katz replied that IS-IS has proven to be very
extensible while still allowing for backward compatibility. IS-IS was also
overhauled around the same time that a lot of ISPs were building their
My questions are these. When is a good time to use IS-IS? Should an ISP
that wants to build a scalable network go with IS-IS right now? Or is
there a certain point where IS-IS becomes desirable over OSPF? Is there
ever such a point? I would think it is better to pick an IGP and stick
Thanks for any comments.
The real truth is that for a very long time cisco's OSPF sucked rocks. Big
rocks. Boulders. Mountains. Entire asteroids. So when certain backbones
asked about new IGP's, I pointed them at IS-IS. It wasn't mature at that
point, but its implementation was head and shoulders above OSPF.
Now, a couple of years later, both the IS-IS and OSPF implementations are
much more stable. Yes, you want to make a careful decision here because
having to change IGPs pretty much causes you a flag day. ;-(
Important points to ponder:
- OSPF has a very strong notion of the backbone area. IS-IS has a slightly
different model with two levels. If your topology means that you'd have to
run all routers as part of the OSPF backbone area, you're not going to be
scalable. Similarly, if you would have to put everything into one IS-IS
area, you're also in trouble. Check out the topology that you can see in
your crystal ball.
- IS-IS is more extensible. It already supports two networks layers and
rumor has it that there's code for another forthcoming. Gotta go to OSPFv3
for this level of extensibility.
- OSPF has a better metric, with enough dynamic range so that you can more
easily achieve "optimal" routing.
- There are more folks running OSPF, so the number of run-hours will grow
faster and the code should be more stable.
- The folks using IS-IS have some of the biggest, most stressed routers in
the world. So I would expect cisco's IS-IS implementation to be more
stressed in the scalability dimension.
So what's best? It depends...