IS-IS
– IS-IS allows a Level-1 Area which is partitioned to be automatically repaired, by electing Partition Designated Level 2 routers and having a virtual link between them. The mechanism is not often implemented and requires an explicit tunnelling mechanism.
– Used in ISO IS-IS for connecting partitions of Level 1 Area over the Level 2 backbone.
OSPF
– Used for connecting physically separate area zeroes (0.0.0.0) to maintain contiguity of the backbone.
– Used for connecting remote areas to the backbone through other areas if direct physical connectivity is not possible. This enables an OSPF packet to be sent from one part of an remote isolated site to the main OSPF network.
– For Virtual links to work, OSPF accepts packets which are have originated more than one hop away. This can lead to security concerns if the packets at the edge of the domain are not properly filtered.
Routing Freak! 