The broadcast keyword provides two functions.
First, it forwards broadcasts when multicasting is not enabled. Second, it
simplifies the configuration OSPF for non-broadcast networks that use Frame
Relay. The broadcast keyword might also be required for
some routing protocols, for example, AppleTalk.
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.
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NOTE:
The OSPF broadcast mechanism assumes that IP class D addresses are
never used for regular traffic over Frame Relay.
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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.