OSPF Network Design Solutions

Configuring an Area Border Router (ABR)

The process for configuring an ABR for OSPF is essentially the same as described in the preceding section “Enabling OSPF on an Inter-Area Router” with just a few minor additions:

1.  Before starting the OSPF routing process, you need to decide on a few things about how OSPF is going to be configured for OSPF in your network. These considerations include: Deciding what OSPF routing process ID you want to assign to your network and deciding if you want OSPF to determine which router becomes the Designated Router (DR) and Backup Designated Router (BDR). The second consideration might require you to decide upon setting a loopback interface. If you do decide to configure a loopback interface then please refer to the section “Creating A Loopback Interface” later in this chapter for specific details.

2.  Turn on the OSPF routing process with the router ospf process-id command as described in the previous section on “Enabling OSPF on an Inter-Area Router.”

3.  Assign the appropriate network statements to the OSPF routing process with the correct area ID, for example:

router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1

4.  Is the area going to be a stub area? If so, enter the area area-id stub [no-summary] command, which defines a stub area. You will also need to enter the area area-id default-cost cost command, which assigns a specific cost. Some additional commands for areas that you might want to consider are covered in Chapter 6.

5.  You will want to add the area range command so that the networks within each area can be properly summarized, for example:

router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1 area 1 range 130.10.8.0 255.255.255.0

6.  Determine if you are going to use any optional OSPF parameters. You do not need to decide now to use any of these options, but be aware of them as they can help your OSPF network. Although many of these have been discussed already, the following list highlights a few of the more significant optional parameters in command syntax:

area area-id authentication area area-id authentication message-digest ip ospf authentication-key ip ospf hello-interval ip ospf dead-interval timers spf spf-delay spf-holdtime

You can use the show ip ospf border-routers command to see the area border routers within your network. This command is explained in more detail in Chapter 8, “Monitoring & Troubleshooting an OSPF Network.”

Configuring an Autonomous System Boundary Router (ASBR)

The process of configuring an autonomous system boundary router (ASBR) for OSPF is very similar to how you would configure an ABR:

1.  You should already know the OSPF Process ID, whether or not you need a loopback interface, and which optional OSPF parameters you are going to be using.

2.  Turn on the OSPF routing process as described previously in the section “Enabling OSPF on a Router.” Again, you will use the router ospf process-id command.

3.  Assign the appropriate network statements to the OSPF routing process with the correct Area ID, for example:

router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1

4.  Then you will want to add the area range command so that the networks within each area can be properly summarized, for example:

router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1 area 1 range 130.10.8.0 255.255.255.0

5.  At this point, you will want to begin the redistribution process between your OSPF autonomous system and the external autonomous system to which the ASBR is providing connectivity, for example:

router ospf 109 redistribute rip subnets metric-type 1 metric 12 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1 area 1 range 130.10.8.0 255.255.255.0 router rip network 128.130.0.0 passive interface s 0 default-metric 5

You can use the show ip ospf border-routers command to see the area border routers within your network. This command is explained in more detail in Chapter 8.

Configuring a Backbone Router

The process of configuring an OSPF backbone router for OSPF is very similar to how you would configure an ABR:

1.  You should already know the OSPF Process ID, whether or not you need a loopback interface, and which optional OSPF parameters you are going to be using.

2.  Turn on the OSPF routing process as described previously in the section “Enabling OSPF on a Router.” Again, you will use the router ospf process-id command.

3.  Assign the appropriate network statements to the OSPF routing process with the correct Area ID, for example:

router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1

4.  Then you will want to add the area range command so that the networks within each area can be properly summarized, for example:

router ospf 109 network 130.10.8.0 0.0.0.255 area 0 network 172.25.64.0 0.0.0.255 area 1 area 1 range 130.10.8.0 255.255.255.0

Configuring a Simplex Ethernet or Serial Interface

Because simplex interfaces between two devices on an Ethernet represent only one network segment, for OSPF you must configure the transmitting interface to be a passive interface. This prevents OSPF from sending hello packets for the transmitting interface. Both devices are able to see each other via the hello packet generated for the receiving interface.

This means that the suppression of sending hello packets is required on the specified interface. This is accomplished using the following command:

passive interface type number