hcb@clark.net (Howard C. Berkowitz) writes:
When I gave my OSPF tutorial at NANOG in June, I stressed OSPF shouldn't be thought of purely as a 2-level hierarchy with a routing domain consisting of an area 0 and a set of nonzero areas. Some of the OSPF scaling problems I see, and these are probably equally likely in IS-IS, come from people trying to put everything into a single OSPF routing domain. Aside from performance issues, this can become a network administration nightmare.
Splitting the interior network into several IGP routing domains, and linking these with a backbone-of-backbones, helps both performance and administration. The backbone group doesn't need to be concerned with LAN installations in a POP or customer site. Depending on the particular network, you might link IGP routing domains with:
-- static routes -- iBGP, putting all IGPs in a single AS -- iBGP and eBGP in a confederation -- Hybrid layer 2/3 techniques, such as linking IGP-routed domains to internal layer 2 "superhubs"
How much IGP support you need will depend on your network. A large enterprise, or a provider of both connectivity and content, will probably need more IGP stuff than a pure connectivity provider.
Howard, Yes, those sound like a list of administrative nightmares. ;-) Wouldn't it be much easier to make use of a three or four level hierarchical IGP? Tony