Hello,

There are lots of details about that..

But the basic concept as follows:

1- ISIS is OSI developed standard Operates at layer-2 while OSPF operates at layer-3.

2- Both represents Link state Protocols.

3- Both represent a Backbone Area, Area 0 in OSPF while Level-2 in ISIS.

4- Both requires hierarchal Design.

5- ISIS uses TLvs while OSPF uses LSAs.

6- Each ISIS router require a unique NSAP Address.

7- Every OSPF Area builds its own link state Database.

8- ISIS builds 2 seperate Dtabases, Level1 Database & Level2 Database.

9- Every ISIS level-1 router (Stub) has a default route toward level-2 to reach another level-1 ISIS router.

10-ISIS TLvs 122, 33 , 135 carries Wide Metric.

11- ISIS illustrates (DIS) in a Multiaccess Segment, There is no Backup DIS , while OSPF illustrates (DR and BDR).

The most Routing protocol Used by the Service Providers is ISIS because Of its simplicity and its Structure maintained throughout different type of Config.. Ie: Redistribution into ISIS doesnt change the main Structure For ISIS protocol while Doing the Same in OSPF changes its main structure.

HTH

Mohamed