From CT3

By Ivan Pepelnjak

IS-IS routing protocol is not well-suited for partially-meshed IP subnets. Contrary to OSPF, which has evolved into a versatile protocol supporting multiple WAN models (broadcast, non-broadcast, point-to-multipoint), IS-IS supports only two types of inter-router connections: point-to-point links and broadcast multi-access subnets (typically used to model LAN interfaces).

The limitations of IS-IS network representation make it particularly hard to use on partially-meshed WAN (Frame Relay or ATM) networks. Generic optimal design uses point-to-point subinterfaces to convert each WAN virtual circuit (or DLCI) into a point-to-point router-to-router link. This design is functionally equivalent to point-to-multipoint OSPF network type but requires more logical interfaces and more complex router configuration. It’s also harder to add new neighbors or PVCs; while OSPF automatically discovers new connections and neighbors when operating in point-to-multipoint mode, you have to configure each subinterface and DLCI manually when using IS-IS.

If you configure IS-IS on multi-access WAN interface (main serial interface or multipoint subinterface), IS-IS models the multi-access interface as a LAN interface (equivalent to OSPF operating in broadcast mode). One of the routers is elected Designated Intermediate System (DIS) and generates Link State Packet (LSP) representing the IP subnet. IS-IS assumes full connectivity between all neighbors on the IP subnet, potentially resulting in unexpected behavior in partially meshed subnets.

More details

• IS-IS on multi-access partially-meshed Frame Relay interface

Related information

• Next-hop fixup in partially-meshed NBMA networks
• Tutorial: OSPF Network Types and Frame Relay
• Tutorial: Point-to-multipoint OSPF over Frame Relay