Configuring OSPF over an NBMA Topology

The design considerations of running OSPF over an NBMA topology are outside the scope of this book, but it is important to research this area because it speaks to the configuration choices that will be made. However, one of the common choices on a partially meshed network illustrates the kind of decisions that need to be made.

If the network is partially meshed, then the choice to use only point-to-point subnets can waste addresses. If a point-to-multipoint configuration is chosen, the network uses one subnet, and there is no DR/BDR negotiation. This has the advantage of saving addresses but behaving as if it is a series of point-to-point links.

When the decision is made as to which technology is to be implemented, the configuration is straightforward. The choice is defined on the interface as a network command. The network command syntax is as follows:

ip ospf network {broadcast | non-broadcast | {point-to-multipoint [non-broadcast]}}

Table 5-6 explains the command. Table 5-6 The ip ospf network Command

Option

Description

nonbroadcast

Sets the network mode to nonbroadcast multiaccess (NBMA mode). This is the

default mode for serial interfaces with Frame Relay encapsulation and point-to-

multipoint subinterfaces.

point-to-multipoint

Sets the network mode to point-to-multipoint

point-to-multipoint

(Optional) Sets the network mode to point-to-multipoint nonbroadcast.

nonbroadcast

broadcast

Sets the network mode to broadcast.

point-to-point

Sets the network mode to point-to-point. This is the default mode for point-to-

point subinterfaces.

For the default encapsulations and hello timers, refer to Table 5-4.

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