Configuring OSPF in NBMA Mode

In NBMA mode, the design considerations are imperative because the selection of the DR and BDR need to have physical connectivity to all routers in the NBMA cloud. Remember that this is a nonbroadcast environment, so the DR and BDR must be configured with a static list of the other routers attached to the cloud so that they can become neighbors and create adjacencies. This is achieved with the use of the neighbor command.

NOTE

Other technologies have superceded the need to manually configure the neighbors.

The syntax of the command is as follows:

neighbor ip-address [priority number ] [poll-interval sec] [cost number]

The neighbor command is used to configure OSPF routers interconnecting to nonbroadcast networks. The different options used with the neighbor command are explained in Table 5-7. Table 5-7 The OSPF neighbor Command

neighbor Command

Description

ip-address

Interface IP address of the neighbor.

priority number

(Optional) An 8-bit number indicating the likelihood of this router being

elected as the BDR or the DR. The default is 0. This keyword does not apply

to point-to-multipoint mode interfaces because no BDR or DR is selected.

poll-interval sec (Optional) Unsigned integer value reflecting the poll interval. RFC 1247

recommends that this value be much larger than the hello interval. The default is 120 seconds (2 minutes). This keyword does not apply to point-to-multipoint mode interfaces.

If a neighboring router has become inactive (hello packets have not been seen for the router dead interval period), it may still be necessary to send hello packets to the dead neighbor. These hello packets will be sent at a reduced rate called the poll interval.

cost number (Optional) Value that assigns a cost or metric. It takes the form of an integer from 1 to 65535. Neighbors with no specific cost configured will assume the cost of the interface, based on the bandwidth or the ip ospf cost command. On point-to-multipoint mode interfaces, this is the only keyword and argument that make sense. This keyword does not apply to NBMA mode networks.

Example 5-2 shows how the command is used. Example 5-2 The OSPF neighbor Command

ROUTER(config)#interface Serial0

ROUTER(config-if)#ip address 131.144.10.100 255.255.255.0 ROUTER(config-if)#encapsulation frame-relay ROUTER(config-if)#ip ospf network non-broadcast ROUTER(config)#router ospf 1

ROUTER(config-router)#network 131.144.10.100 0.0.0.255 area 0 ROUTER(config-router)#neighbor 131.144.10.2 ROUTER(config-router)#neighbor 131.144.10.3 ROUTER(config-router)#neighbor 131.144.10.5

NOTE

NBMA mode is used by default, so there is no need for the ip ospf network non-broadcast command. However, neighbor statements are necessary.

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