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| 4.6 |  | PPP and Layer 2 Considerations for Routed and Routing Protocols |
| 4.6.6 |  | The keyword broadcast |

The broadcast keyword provides two functions. First, it forwards broadcasts when multicasting is not enabled. Second, it simplifies the configuration of Open Shortest Path First (OSPF) for non-broadcast networks that use Frame Relay.

The broadcast keyword might also be required for some routing protocols, for example, AppleTalk. Protocols such as these depend on regular routing table updates, especially when the router at the remote end is waiting for a routing update packet to arrive before adding the route.

By requiring the selection of a designated router, OSPF treats a NBMA network such as Frame Relay in much the same way as it treats a broadcast network. In previous releases, this required manual assignment in the OSPF configuration using the neighbor interface router command. When the frame-relay map command is included in the configuration with the broadcast keyword, and the ip ospf network command with the broadcast keyword is configured, there is no need to configure any neighbors manually. OSPF now automatically runs over the Frame Relay network as a broadcast network.

> Note: The OSPF broadcast mechanism assumes that IP class D addresses are never used for regular traffic over Frame Relay.

The following example maps the destination IP address 172.16.123.1 to DLCI 100:

> Router(config)#interface serial 0/0
> Router(config-if)#frame-relay map IP 172.16.123.1 100 broadcast

OSPF uses DLCI 100 to broadcast updates.