By default, OSPF messages are forwarded out all OSPF-enabled interfaces. However, these messages really only need to be sent out interfaces connecting to other OSPF-enabled routers.
Refer to the topology in the figure. OSPF messages are forwarded out of all three routers G0/0 interface even though no OSPF neighbor exists on that LAN. Sending out unneeded messages on a LAN affects the network in three ways:
- Inefficient Use of Bandwidth - Available bandwidth is consumed transporting unnecessary messages. Messages are multicasted; therefore, switches are also forwarding the messages out all ports.
- Inefficient Use of Resources - All devices on the LAN must process the message and eventually discard the message.
- Increased Security Risk - Advertising updates on a broadcast network is a security risk. OSPF messages can be intercepted with packet sniffing software. Routing updates can be modified and sent back to the router, corrupting the routing table with false metrics that misdirect traffic.