A quick peek at "Intermediate System-Intermediate System" aka ISIS

In this blog, we will look at the concepts and a few difference of IS-IS and OSPF.

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 224.0.0.5/6 )
• 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

Also you can see in the above diagram, since the extended core has no area0, it's considered a loose core , that can be added on with no rigid requirements as in OSPF,  and  with all areas attached to a  common backbone ( area 0 )

Where ISIS shines, if you  merge or gobble up another network, it's easier to integrate that newly acquired network via ISIS vrs OSPF.

Ken Felix
Freelance Network/Security Engineer
kfelix at hyperfeed  dot com