1st Here 's a a typical view of the 2 types of network with 4 areas;
As you can see in the bottom halve, all areas are connected to Area0 with a ABR. If you have a big network, this area0 can be swamped with LSA, and the ospf-database will become massive.
ISIS overcomes this requirement, since all of the sub-areas within ISIS, does NOT need connectivity to a common core area0. Each area is a subdomain , and can be connected to each other via a level1/2 router. Each router within the area is a Level1 and only knows about it's self and the other Level1s in that area and the exist via the Level1/2s. Also with ISIS we now can travel directly from one area to another if we have connectivity to that area.
NOTE: The level1-2 routers are truly similar to a OSPF abr.
A few of differences that you need to understand about ISIS;
- It does not need or use virtual-links it is a IS-IS network ( pun intended :) )
- It support ipv6 and dynamic routing
- It supports both md5 and clear-text authentication
- SPF calculation via Dijkstra alogrithm
- ISIS is encapsulated in layer2 ( no multicast group like that of ospf 188.8.131.52/6 )
- ISIS == more for the ISP
- OSPF == more for the Enterprise and the smaller ISP
- Does not have a direct sub-routing option like that of M-OSPF ( multicast OSPF )
- A router could be label as a level1 or 2 or 1-2
- does not support NBMA or multi-point topologies
Where ISIS shines, if you merge or gobble up another network, it's easier to integrate that newly acquired network via ISIS vrs OSPF.
Freelance Network/Security Engineer
kfelix at hyperfeed dot com