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

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By Ivan Pepelnjak

When you configure IS-IS on a multi-access WAN interface (a serial interface configured with Frame Relay encapsulation, an ATM interface or a multipoint subinterface), it models the WAN interface as a broadcast multi-access LAN network. IS-IS assumes that:

  • It can use broadcasts to discover neighbors with the IS-IS hello messages;
  • A Designated Intermediate System (DIS) will be elected to represent the IP subnet as a network LSP.
Running IS-IS on multi-access WAN interface is almost identical to running OSPF in broadcast mode.

Contrary to OSPF operating in broadcast mode, Cisco IOS implementation of Integrated IS-IS does not assume that all routers connected to the IP subnet can reach each other directly, but computes correct IP next-hops based on actual neighbor adjacencies.

If you want to use IS-IS on a partially-meshed WAN network, you have to make sure that:

  1. Broadcasts get propagated across all virtual circuits.
  2. The router that is elected DIS has direct connectivity to all other routers connected to the same subnet.

To satisfy these requirements, use the following configuration steps:

  • Configure each Frame Relay PVC manually on the multi-access interface with the frame-relay map clns dlci broadcast interface configuration command.
A Frame Relay map has to be configured for each DLCI you want to use. IS-IS cannot auto-discover new DLCIs.
  • Increase the DIS election priority of the hub router(s) with the isis priority number interface configuration command. Set the DIS election priority on non-hub routers to zero to ensure they can never become DIS.
Election of a DIS that does not have direct DLCIs to all other routers in the same subnet results in partial connectivity over the WAN subnet.
IS-IS generates correct next-hops based on actual IS-IS adjacencies. No extra configuration steps are necessary to establish end-to-end connectivity once the IP routing table is computed.

More details

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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