As far as OSPF, I know that Cisco recommends that you should not have an OSPF area with more than 90-100 routers. Is there such restriction or recommendation for ISIS areas or levels?

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In theory, just like for OSPF, there is no limit. There will be a practical limit, based on the routing protocol traffic generated, bandwidth, and router resources.

Some people have networks that work with 1000 routers for either OSPF or IS-IS, but that would be rare, and it is probably a network that needs to be redesigned. I actually worked on one, and it took over an hour to converge. Eventually, it was broken up into ASes, using BGP between the ASes, and OSPF withing the ASes.

In practical terms, you will probably see no more than a couple of dozen routers in any OSPF or IS-IS area. More than that would probably be symptomatic of a network that needs to be redesigned.

All the areas, except the backbone are on the same level, so you can have an almost unlimited number of areas at the same level, but they will be separate areas.

  • Gotcha! So, in real world scenario, I am assuming a typical design of a backbone network using ISIS would be: multiple areas on same level 2. I think using two different levels is avoided as far as possible to keep the network simple. This will allow expansion of the network by adding more areas in the same level and each area having upto 30 routers.
    – monica
    Feb 18, 2018 at 22:56
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    Using different areas with a backbone can give you the opportunity to aggregate addressing for an area, and only advertise a summary address for an entire area. This reduces the router resources required, and it shrinks routing tables (allowing one routing table entry for an entire area in other areas, instead of dozens or hundreds of routing table entries).
    – Ron Maupin
    Feb 18, 2018 at 23:02
  • There are ABRs in ISIS. So who does the summarization in this case?
    – monica
    Feb 19, 2018 at 0:52
  • You summarize at the area boundary, so on the L1/L2 router (OSPF ABR).
    – Ron Maupin
    Feb 19, 2018 at 1:03
  • That means, L1/2 routers will summarize routes for each area present in that level and advertise to another level. But all the router in the same level will carry all routes of all the routers in all the areas in that level. Is that correct?
    – monica
    Feb 19, 2018 at 1:25

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