Scaling IS-IS
Currently, IS-IS is being used as an IGP by some of the largest ISPs. In most cases, a well-defined ISP network should not have a large IGP routing table, but due to extensive redundancy, scaling does become a problem. In addition, even if the IGP has a strong addressing structure, sometimes it must find specific routes to the next hop according to strict policy requirements. For this reason, route summarization is not always possible.
Experience in working with IS-IS has provided some insight that may be useful to you. One of the key things to remember is that Cisco defaults to both the level 1 and level 2 routers because all the level 2 routers must route within their area. In addition, the router cannot distinguish whether ...
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