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New Member

IS-IS Network Design

Can someone give me a helping hand?

I am using cisco 2620 routers (provisioning: 2x Smart Serial / 1x Ethernet / 16 MB Flash and default RAM)in a pure DCN (means: only clnp traffic)environment. All routers are running IS-IS. The network is separated into areas. Are there any concret rules concerning

- the maximum number of entries (nodes)in a level 1 database?

- the maximum number of entries (nodes) in the level 2 database?

- the maximum number of adjacencies?

Thanks in advance



Re: IS-IS Network Design

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Re: IS-IS Network Design

There aren't any concrete numbers for any of these, unfortunately.... It depends on what each router is doing/etc. We have seen level 1 and level 2 routing domains with hundreds of routers in each, as a point of reference.


New Member

Re: IS-IS Network Design


Do you have any white paper or url which explain about IS-IS Network Design ? I need those info to set up IS-IS in our network. Thank you very much.

Duc Vu

New Member

Re: IS-IS Network Design

In IP networks, with 7200's and larger it is very common to see networks with hundreds of IS's in a single L2 environment.

However, in the DCN environment, we use a multi-level architecture and try to keep IS counts down under 50 and often under 25. This is because some IS-IS implementations running on SONET NEs which we must interoperate with were not designed to support huge topologies. This is typically in the L1 areas.

In the L2 backbone, the device count can be much higher, depending on which routers are used. It is often suggested to make use of the 3662 series routers and the IS-IS MA feature, allowing a number of L1 areas to be terminated on the same router. Typically we do not suggest more than around 12 L1 areas to be terminated on a single router in an IS-IS MA environment.

When IS-IS L2 environment reach a point at which they need to be broken up, we have historically used ISO-IGRP to connect them, however with CLNS support being added to BGP, that will hopefully take the place of ISO-IGRP in this environment.

Hope this helps...