The OSPF area range Command for an ABR

The area range command is configured on an ABR because it dictates the networks that will be advertised out of the area.

Use the area router configuration command with the range keyword to consolidate and summarize routes at an area boundary. Use the no form of this command to disable this function for the specified area:

area area-id range address mask no area area-id range address mask

In the preceding syntax, area-id is the identifier (ID) of the area about which routes are to be summarized. It can be specified as either a decimal value or an IP address. Here, address is the IP address, and mask is the IP mask.

Figure 6-7 illustrates the example configuration that follows. Figure 6-7 The OSPF area range Command for an ABR

Figure 6-7 illustrates the example configuration that follows. Figure 6-7 The OSPF area range Command for an ABR

Example 6-2 shows the configuration required to summarize the following five individual subnets (which can address six hosts each) into one subnet. This summarized subnet may then be propagated across the OSPF network, saving both bandwidth and CPU:

These subnets are summarized into one subnet:

This one subnet will then be propagated into Area 1.

Example 6-2 The OSPF area range Command for an ABR

Router Jack router ospf 100

network 172.16.20.128 0.0.0.7 area 0

network 172.16.20.8 0.0.0.7 area 1

area 0 range 172.16.20.128 255.255.255.192

NOTE The area ID requested is the area that the subnets originated from. It is not the destination area.

The summarization update populates the topological databases of the routers in the destination area. These routers will need to know the source area for the summarized subnet to know where to send the data traffic.

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