OSPF Network Design Solutions

Configuring OSPF for Broadcast or Nonbroadcast Multiaccess Networks

You have the choice of configuring your OSPF network type as either broadcast or nonbroadcast multiaccess (X.25), regardless of the default media type. For example, it does not matter if you have a broadcast media type, such as Ethernet, because you can still configure it as nonbroadcast if you so desire.

Using this feature, you can configure broadcast networks as nonbroadcast multiaccess networks when, for example, you have routers in your network that do not support multicast addressing.

You also can configure nonbroadcast multiaccess networks (such as X.25, Frame Relay, and SMDS) as broadcast networks. This feature saves you from having to configure neighbors, as described in the section “Configuring OSPF for Nonbroadcast Networks,” later in this chapter.

Why would it be beneficial not to have a neighbor? This is a very odd statement because OSPF utilizes neighbors quite extensively. Assume, for example, that you have a point-to-point network. By not using neighbors you can reduce router memory and processor usage since there is only one other router to talk with.

Configuring OSPF for Nonbroadcast Networks

Because there might be many routers attached to an OSPF network, a designated router is selected for the network. You must use special configuration parameters in the designated router selection if the broadcast capability is not configured.

These parameters need only be configured in those devices that are eligible to become the Designated Router (DR) or Backup Designated Router (BDR).

To configure routers that connect to nonbroadcast networks, you can specify the following neighbor parameters, as required:

•  Priority value for a neighboring router

•  Nonbroadcast poll interval

•  Interface (via IP Address) through which the neighbor is reachable

These features enable you to determine several OSPF operating variables in just one router that will be propagated to its neighbors that are identified in the following configuration command:

neighbor ip-address [priority number] [poll-interval seconds]

Configuring OSPF for Point-to-Multipoint Networks

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface with the router having one or more OSPF neighbors. Because of this, OSPF will create multiple host routes.

An OSPF point-to-multipoint network has the following benefits compared to nonbroadcast multiaccess 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.

•  Point-to-multipoint has fully-meshed topology.

•  Point-to-multipoint is more reliable because it maintains connectivity in case of virtual circuit failure.

When you decide to configure nonbroadcast, multiaccess networks as either broadcast or nonbroadcast networks, OSPF assumes that there are virtual circuits from every router to every router or that you are running a fully-meshed network.

This is not true in many cases because you might only have a partially-meshed network because the cost required to fully mesh the network is prohibitive. In this case, 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 virtual circuits to both routers.

To configure your OSPF network type on a specific interface (int s0), enter the following command in interface configuration mode:

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

Figure 7-19 shows an example of OSPF in a point-to-multipoint networking environment.

Figure 7-19  Point-to-multipoint network example.

Referring to the setup in Figure 7-19 and for demonstration purposes, assume the following scenario:

•  Matt uses DLCI 201 to communicate with Wayne, DLCI 202 to Ben, and DLCI 203 to Robin.

•  Wayne uses DLCI 101 to communicate with Matt and DLCI 102 to communicate with Robin.

•  Robin communicates with Wayne (DLCI 401) and Matt (DLCI 402).

•  Ben communicates with Matt (DLCI 301).

Given this setup, the configurations for Matt, Wayne, Robin, and Ben would be as follows:

Matt’s Configuration

hostname Matt ! interface serial 1 ip address 10.0.0.2 255.0.0.0 ip ospf network point-to-multipoint encapsulation frame-relay frame-relay map ip 10.0.0.1 201 broadcast frame-relay map ip 10.0.0.3 202 broadcast frame-relay map ip 10.0.0.4 203 broadcast ! router ospf 1 network 10.0.0.0 0.0.0.255 area 0

Wayne’s Configuration

hostname Wayne ! interface serial 0 ip address 10.0.0.1 255.0.0.0 ip ospf network point-to-multipoint encapsulation frame-relay frame-relay map ip 10.0.0.2 101 broadcast frame-relay map ip 10.0.0.4 102 broadcast ! router ospf 1 network 10.0.0.0 0.0.0.255 area 0

Robin’s Configuration

hostname Robin ! interface serial 3 ip address 10.0.0.4 255.0.0.0 ip ospf network point-to-multipoint encapsulation frame-relay clockrate 1000000 frame-relay map ip 10.0.0.1 401 broadcast frame-relay map ip 10.0.0.2 402 broadcast ! router ospf 1 network 10.0.0.0 0.0.0.255 area 0

Ben’s Configuration

hostname Ben ! interface serial 2 ip address 10.0.0.3 255.0.0.0 ip ospf network point-to-multipoint encapsulation frame-relay clockrate 2000000 frame-relay map ip 10.0.0.2 301 broadcast ! router ospf 1 network 10.0.0.0 0.0.0.255 area 0