Cisco Systems 78-11741-02 Configuring Ospf over Different Physical Networks, Network segment

Configuring OSPF

Configuring OSPF over Different Physical Networks

Command	Purpose
Router(config-if)#ip ospf message-digest-key	Enables OSPF MD5 authentication. The values for the key-idand
key-id md5 key	key arguments must match values specified for other neighbors on a
	network segment.
Router(config-if)#ip ospf authentication	Specifies the authentication type for an interface.
[message-digest null]

Configuring OSPF over Different Physical Networks

OSPF classifies different media into the following three types of networks by default:

•Broadcast networks (Ethernet, Token Ring, and FDDI)

•Nonbroadcast multiaccess (NBMA) networks (Switched Multimegabit Data Service (SMDS), Frame Relay, and X.25)

•Point-to-point networks (High-Level Data Link Control [HDLC], PPP)

You can configure your network as either a broadcast or an NBMA network.

X.25 and Frame Relay provide an optional broadcast capability that can be configured in the map to allow OSPF to run as a broadcast network. Refer to the x25 map and frame-relay map command descriptions in the Cisco IOS Wide-Area Networking Command Reference publication for more detail.

Configuring Your OSPF Network Type

You have the choice of configuring your OSPF network type as either broadcast or NBMA, regardless of the default media type. Using this feature, you can configure broadcast networks as NBMA networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure NBMA networks (such as X.25, Frame Relay, and SMDS) as broadcast networks. This feature saves you from needing to configure neighbors, as described in the section “Configuring OSPF for Nonbroadcast Networks” later in this chapter.

Configuring NBMA, multiaccess networks as either broadcast or nonbroadcast assumes that there are virtual circuits (VCs) from every router to every router or fully meshed network. This is not true for some cases, for example, because of cost constraints, or when you have only a partially meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network. Routing between two routers not directly connected will go through the router that has VCs to both routers. Note that you need not configure neighbors when using this feature.

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. It creates multiple host routes. An OSPF point-to-multipoint network has the following benefits compared to NBMA and point-to-point networks:

•Point-to-multipoint is easier to configure because it requires no configuration of neighbor commands, it consumes only one IP subnet, and it requires no designated router election.

•It costs less because it does not require a fully meshed topology.

•It is more reliable because it maintains connectivity in the event of VC failure.

Cisco IOS IP Configuration Guide

IPC-226