IS-IS on multi-access partially-meshed Frame Relay interface
From CT3
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:
- Broadcasts get propagated across all virtual circuits.
- 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.