System information

Configuring OSPF

11-23

MOT(config-ospf)#passive-interface {cable | ethernet | gigaether | pos | serial}

where:

cable is the cable interface.

ethernet is the Ethernet/Fast Ethernet interface.

gigaether is the Gigabit Ethernet interface.

pos is the Packet over SONET interface.

serial is the Serial interface.

slot is the module slot number.

interface is the interface number.

Forcing Router ID Choice with Loopback Interface

OSPF uses the largest IP address configured on the interfaces as its router ID. If the

interface associated with this IP address goes down, or if the address is removed, the

OSPF process must recalculate a new router ID and send again all its routing

information.

If a loopback interface is configured with an IP address, the software uses this IP

address as its router ID, even if other interfaces have larger IP addresses. Since

loopback interfaces never go down, this provides greater stability in the routing table.

OSPF automatically prefers a loopback interface over any other kind, and it chooses

the highest IP address among all loopback interfaces. If no loopback interfaces are

present, the highest IP address in the router is chosen.

To configure an IP address on a loopback interface, use the following commands:

1. To create a loopback interface and move to Interface Configuration mode, use the

interface loopback command in Global Configuration mode, as shown below:

MOT(config)#interface loopback 1

2. To assign an IP address to this interface, use the ip address command in Cable

Interface Configuration mode, as shown below:

MOT(config-if)#ip address <A.B.C.D> <A.B.C.D>