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Multiarea ISIS design question


Can someone please help with my ISIS design questions.



The topology is hub and spoke

The hub is a pair of routers connected via an E1, we then have 4 pair's of routers (Gateway routers) covering the south,london, midlands and the north. Each router within a pair is then connected back to one of the hub routers via an E1, each pair of gateway routers are connected via an E1.

The hub and gateway routers are all L2-only for routing within the backbone, each gateway router is running multiple L1 areas for each SDH ring it's managing.

All my L2 routers are in one area (area 10) and are routing IP and OSI

Is the above a good / bad design?

Please comment on the following quote. Do you agree with this comment?


If you configure systems to be L2 only, then you should assign each box into its unique area.



Re: Multiarea ISIS design question

Yes, if the "backbone" is going to be pure L2, then each router needs to be in a different area. I probably wouldn't do this... Instead, I'd make the "backbone" an L1 area, with L2 overlaying on top of it, to simplify things. IS-IS isn't like OSPF in this regard--most people who work with OSPF for a long time want to make a "pure backbone" area like OSPF has. But, if you equate IS-IS' L1 routing with intra-area in OSPF, and L2 with interarea in OSPF, rather than trying to make the ABR the focus of attention, you can see that within area 0 there's still intra-area routing, it's not a "pure barckbone."

So, in IS-IS, you're just being more explicit about your intra-area routing. The main difference is that you have to think of a bunch of L1 routing domains with an overlaying L2 domain providing interconnectivity, rather than a bunch of areas with borders. The L2 backbone isn't a seperate physical entity, it's a seperate logical entity overlayed on top of the same physcial layout as the L1 areas under it.

Take a look at IS-IS Deployment in IP Networks, on Addison Wesley.


New Member

Re: Multiarea ISIS design question

Hi Russ

Thanks for your quick response.

Just to summarise your response

Option 1


Configure each router in my pure L2 "backbone" in a different (not recommended, can you please expand on this)

Option 2


Make my pure L2 "backbone" a L1/L2

p.s. I'm about to order a copy of the IS-IS Deployment in IP networks. Looking forward to reading it, I've found it difficult to get good info on this subject

Thanks for your help


Re: Multiarea ISIS design question

You have the options right.... I don't think I'd configure each router in the "backbone" in a different area just because it seems more difficult to me to do. It's just a matter of perception, really, having to create a new area address, and the to think of it in terms of "strict borders."

It's probably just a comfort level thing, and what I'm used to troubleshooting, etc. You'll be fine either way--there's plenty of precendent either way, as well (I've seen very large networks done both ways).