Running IS-IS over partially-meshed Frame Relay

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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.

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Related information

Additional Resources  

Configuring Configuring Cisco Routers for Integrated IS-IS (CISIS) course:

Building Scalable Cisco Internetworks (BSCI) course:

Other links:

Did you know?

  • NIL developed the first commercially available advanced technical training covering Cisco IOS implementation of IS-IS.
  • This course was for several years the only course available to Cisco's internal audiences and its Service Provider customers in Europe.
  • The IS-IS course developed by NIL later became part of Cisco's Service Provider training curriculum.
  • NIL's experts have helped several large Service Providers design and deploy IS-IS networks.
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