See my earlier comments to Chuck.
>So some people have said that IS-IS is more scalable because it doesn't run
>Dijsktra as much as OSPF does. OK, then why not? Is it because of the
>partial-routing update thing, or is there more to it?
True, both run Dijkstra, but ISIS allows longer timer settings of the
equivalent to MaxAge. Thus, less frequent recomputations in a stable
network, which is far more characteristic of a carrier than an
enterprise network.
And some carriers do run OSPF, for reasons ranging from feature
support to simple familiarity. There are a lot of very poorly
documented extensions to ISIS.
>
>Also, I agree that IS-IS level-1 areas are by their nature "totally stubby".
>But that doesn't completely explain why real-world IS-IS networks have been
>shown to be more scalable than real-world OSPF networks, because if this was
>the cause, then it would seem to me that you could just scale OSPF to the
>same level of IS-IS just by making non-backbone areas totally stubby.
I'm not exactly sure that the scalability argument is being made
correctly. ISIS is definitely more scalable in single areas. Most
providers run it as a single area. With the more extensive controls
at the backbone to nonbackbone boundary, OSPF may very well be more
scalable in more heterogeneous networks.
> Yet,
>apparently nobody has been able to scale OSPF like that, which indicates to
>me that it's not that simple.
>
>So I must ask again, what exactly is it about IS-IS that seems to make it
>more scalable overall than OSPF? And, as a side question, could OSPF be
>reasonably adjusted to accomodate greater scalability?
>
>I would especially like to invite Howard Berkowitz, aka Sir Network Deity,
>to answer this question.
In any case, there are many more factors of development interest than
the number of routers per area. One carrier interest is getting IGP
reconvergence time into the millisecond range, which will require, at
least, much faster hellos, an update or replacement for Dijkstra, and
probably restrictions on routes per area.
Traffic engineering is also an issue -- the work is further along
with ISIS, but that's simply been a matter of development priority.
Some OSPF features that ISIS doesn't have include demand circuits,
native NBMA support, external routers outside the backbone, virtual
links, and the opaque LSA.
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