>Hello all:
>
>Thank you for everybody who answered my previous IS-IS question.
>
>I have been compiling a preliminary list on the advantages and disadvantages
>of IS-IS vs. OSPF, and where you might want to use one over the other. This
>is what I have come up with
Radia Perlman groupies like ISIS.
John Moy groupies like OSPF.
:-)
>
>IS-IS:
>- has a foothold in ISP's for historical reasons, as it was developed
>earlier than OSPF. Therefore, for backwards compatibility, ISP's continue
>to demand routers that can do IS-IS
It's not just historical; ISIS has evolved and been tuned to meet
quite a number of ISP needs. Unfortunately, much of this is extremely
poorly documented, either in RFCs or even in vendor manuals.
ISIS routinely supports far more routers than OSPF in a single area,
and many ISPs, at least in their core, are single areas. 1000+
ISIS speaking routers are routine.
ISIS, especially with tuning, requires less bandwidth than OSPF,
especially in the stable optical networks of carrier cores.
>- also is used for out-of-band SDH management by telcos
>- Converges slightly faster than OSPF
I have seen no evidence for this.
>- (Naturally) is multiprotocol, so can handle CLNS, and Decnet phase V
>(which is CLNS)
Remember at the time of its introduction, OSI vs TCP/IP was still
a very open issue. In 1990-1991, when the first major ISPs were
being implemented, CLNP support was an important business concern.
Multiprotocol is far less of an issue these days. The world is moving
to IP, with tunneling of legacy protocols. IPv4-IPv6 evolution is
being planned quite carefully.
>- Has some features that OSPF does not that can be useful in special
>situations, like the OL bit, etc.
Has a lead in the implementation of traffic engineering.
I'm not sure that the OL bit or the equivalent OSPF database overflow
feature have ever become that important, because iBGP wasn't well
understood at the time of the design of both these IGPs.
>
>OSPF
>- Is better known, and documentation for it is more readily available
>- Has an overall richer set of features than IS-IS (at least, until the
>latest IS-IS revisions)
It's probably most precise to say that OSPF has the ability to
have much greater control over what deliberately leaks between
areas, and between the OSPF domain and other domains. This may
be useful in ISP POPs, but is more important for enterprises that
have distinct communities of interest (common applications).
>- Is the standard link-state routing protocol for enterprises, and is also
>popular in ISP's.
Using the opaque LSA, is more flexible for uses that were not
necessarily
thought about at the time of development, which might very well become
useful for things like sub-IP control. ISIS does allow the creation
of new TLVs, but that has to be done at a code level.
>
>Does anybody have anything to add?
>
>
>Also, I would like to know what people think the future of IS-IS is,
>particularly after the latest revisions. Does anybody think that IS-IS will
>be able to maintain and expand its foothold in ISPs, and even move to the
>enterprise, or is it forever doomed to its niche (and why do you think so)?
I'd hesitate to use the term "doomed." Large ISPs have different
requirements than enterprises, so why should the same fundamental
protocol be ideal for both of them? Things become much more complex
when considering things like content providers.
Both protocols have limitations and it isn't a given that
significantly new versions, or even new IGPs, may come along. ISPs,
in particular those that offer voice services, are very interested in
subsecond convergence. That isn't going to happen without rethinking
failure detection, and probably replacing the 40-year-old Dijkstra
algorithm with a faster link state algorithm.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=4972&t=4945
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