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RE: IS-IS use?? posted 11/17/2000
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Don't get me wrong. There are definite times to use ISIS, there are definite times to use OSPF, and there are times either will work just fine.

Russ Meyer <rmeyer@xxxxxxxxx> wrote

IS-IS is considered more stable because you can manually set the refresh
time as far out as 18 + hours.

This is desirable when the underlying routers and links are stable, which is the typical case inside a provider network. A widely distributed enterprise network, however, may need a reality check more frequently.


Also, it is more secure, because routing
updates are transmitted via a layer 2 transport, (CLNS).

True, ISIS runs over the data link layer (not CLNP). In one sense, you can argue that is more secure because updates are limited to one link.


But OSPF and EIGRP have their IP hop count set to 1, so that will limit them to one link as well.

Also, IS-IS can
support over 850 routers in a single area.

The more basic question is when is it desirable to support that many routers in a single area? It can be appropriate in some provider networks, but it tends to be a terrible idea in enterprise networks. That being said, there's at least one highly tuned OSPF network that runs 1000 or so routers in area 0.0.0.0, generally an insane idea. Most carriers using ISIS, according to Dave Katz, use a single area.


Try doing that with OSPF, also
most enhancements that are added to OSPF are also added to IS-IS.

Basic ISIS non-backbone areas behave like OSPF totally stubby areas. Obviously, there are cases in which totally stubby areas are very useful. It's only recent, however, that there's starting to be controlled L2 injection into ISIS level 1 areas. Without such a feature, you are limited to a cold potato/closest exit routing policy.

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