OSPF vs ISIS - Which do you prefer & why?

Mark Tinka mark.tinka at seacom.mu
Thu Nov 10 06:52:59 UTC 2016

On 10/Nov/16 08:41, Wayne Bouchard wrote:

> This generally supports my own view that it depends on the topology
> and the real or potential scale/scope. In my experience, IS-IS is just
> all around better in a flat, highly interconnected environment such as
> an ISP or other broadly scaled network. If you have a very (almost
> exclusively) heirarchical structure and pretty good control over IP
> addressing and can use summarization effectively, then OSPF can make
> your core networking much simpler. On a small network that doesn't
> look to grow at leaps and bounds, I'd favor OSPF. On a large, complex
> network or a network that has the potential to grow without any sort
> of predefined structure (ie, more demand based), then IS-IS is
> probably your win.

I wouldn't base the choice necessarily on the size of the network,
although from experience, I'd always choose IS-IS for a large,
geographically-dispersed network.

What I mean to say is that I'd be happy running IS-IS on a tiny network,
as much as I'd run route reflectors in a 3-router network. There isn't
that much more effort to get it going compared to considering whether a
Mini or a dump truck should be used to take the kids to school every

>  Note that this doesn't factor in multiple IS-IS
> levels, something I don't have a great deal of experience with.
> Mostly, networks I've been associated with just run one great big,
> gigantic level 0, though they did also experiment with other
> configurations.

I've run multi-level IS-IS before. To be honest, I'd not recommend it.

There are enough features and knobs in IS-IS to quiet the chatter
associated with a single-level IS-IS deployment.

Running multi-level IS-IS means you need to plan your L1/L2
intersections, figure out what to do with the ATT Bit and look at Route
Leaking if you run MPLS (LDP hates route summarization).

Needless to say, the Area ID of the NET is more significant in L1/L2
IS-IS than in L2-only IS-IS, as this is what is used to control LSP
exchange between levels. This can get very complicated very fast in very
large networks. Yep, that's 3 "very's".

It's 2016, and any decent router has something-x86 going for it (or at
the very least, a reasonably quick non-x86 going for it). I'd stick to
single-level IS-IS. Which means L2-only, not L1-only - I know a network :-).

My only real issue with IS-IS is the ST and MT (Single Topology and
Multi-Topology) nuisance re: IPv6. Many vendors implement ST on turn-up,
meaning you need to manually configure the MT knob. This can be painful
when you haven't been clued up on MT, run an IPv4-only network and need
to enable IPv6. I've gone into a bit of detail on this in my talk that I
included in a previous post.


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