Cisco CCNP / BSCI Exam Tutorial: Using The OSPF Command “Area Range”

Your BSCI and CCNP exam success depends on knowing the details, and one such detail is knowing the proper way to summarize routes in OSPF. Route summarization is not just a test of your binary conversion abilities, but knowing where and when to summarize routes. It will not surprise any CCNA or CCNP certification candidate that OSPF gives us the most options for route summarization, and therefore more details to know!

OSPF offers us two options for route summarization configurations. In a previous tutorial, we looked at the "summary-address" command, and today we'll look at the proper use of the "area range" command.

The "area range" command should be used on an Area Border Router (ABR) to summarize routes being advertised from one OSPF area to another. In this tutorial, R1 is acting as an ABR, with interfaces in both Area 0 and Area 1. Four loopbacks have been placed into R1's Area 1.

R1(config)#router ospf 1

R1(config-router)#network 12.0.0.0 0.255.255.255 a 1

R1(config-router)#network 13.0.0.0 0.255.255.255 a 1

R1(config-router)#network 14.0.0.0 0.255.255.255 a 1

R1(config-router)#network 15.0.0.0 0.255.255.255 a 1

The routing table of an OSPF neighbor, R2, shows all four routes.

R2#show ip route ospf

12.0.0.0/32 is subnetted, 1 subnets

O IA 12.12.12.12 [110/65] via 172.12.123.1, 00:18:52, Serial0

13.0.0.0/32 is subnetted, 1 subnets

O IA 13.13.13.13 [110/65] via 172.12.123.1, 00:18:42, Serial0

14.0.0.0/32 is subnetted, 1 subnets

O IA 14.14.14.14 [110/65] via 172.12.123.1, 00:18:32, Serial0

15.0.0.0/32 is subnetted, 1 subnets

O IA 15.15.15.15 [110/65] via 172.12.123.1, 00:18:32, Serial0

To keep the routing tables of downstream routers smaller but still have the desired IP connectivity, we can use the area range command on R1 to summarize these four routes. The key to keep in mind with the area range command is that the area number given in the command is the area containing the destinations, NOT the area that will receive the summary route.

R1(config)#router ospf 1

R1(config-router)#area 1 range 12.0.0.0 252.0.0.0

R2 now shows a single summary route that can be used to reach all four remote networks.

R2#show ip route ospf

O IA 12.0.0.0/6 [110/65] via 172.12.123.1, 00:00:21, Serial0

Interestingly enough, there's now an additional route in R1's routing table.

R1#show ip route ospf
O 12.0.0.0/6 is a summary, 00:07:53, Null0


When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table of the router performing the summarization. This helps to prevent routing loops. Any packets destined for the routes that have been summarized will have a longer match in the routing table, and packets that do not match one of the summarized routes but do match the summary route will be dropped.

CCNP / BSCI Exam Tutorial: Route Summarization And The OSPF Null Interface

CCNP exam success, particularly on the BSCI exam, demands you understand the details of route summarization. This skill not only requires that you have a comfort level with binary conversions, but you have to know how and where to apply route summarization with each individual protocol.

You also have to know the "side effects" of route summarization. With OSPF, there will actually be an extra interface created at the point of summarization, and this catches a lot of CCNP candidates by surprise. Let's take a look at the null0 interface and how it relates to OSPF summarization.

On R1, the following networks are redistributed into OSPF, and then summarized.

interface Loopback16

ip address 16.16.16.16 255.0.0.0


interface Loopback17

ip address 17.17.17.17 255.0.0.0


interface Loopback18

ip address 18.18.18.18 255.0.0.0

interface Loopback19

ip address 19.19.19.19 255.0.0.0

R1(config)#router ospf 1

R1(config-router)#redistribute connected subnets

R1(config-router)#summary-address 16.0.0.0 252.0.0.0

The summary address appears on R2, a downstream router.

R2#show ip route ospf


O E2 16.0.0.0/6 [110/20] via 172.12.123.1, 00:00:05, Serial0


Let's go back to R1 and look at its OSPF table.

R1#show ip route ospf


O 16.0.0.0/6 is a summary, 00:01:51, Null0

Where did the null0 interface come from, and why is it there? Packets sent to the null interface are dropped, and in this case, that's a good thing.

When you configure summary routes in OSPF, a route to null0 will be installed into the OSPF routing table. This helps to prevent routing loops. Any packets destined for the routes that have been summarized will have a longer match in the routing table, as shown below...

C 17.0.0.0/8 is directly connected, Loopback17

C 16.0.0.0/8 is directly connected, Loopback16

C 19.0.0.0/8 is directly connected, Loopback19

C 18.0.0.0/8 is directly connected, Loopback18

O 16.0.0.0/6 is a summary, 00:01:51, Null0


.. and packets that do not match one of the summarized routes but do match the summary route will be dropped.

Preventing routing loops when performing route redistribution and summarization is vital. OSPF gives us a little help in that regard in this situation, and as you study more complex redistribution scenarios on your way to the CCNP and CCIE, you'll realize that we'll take all the help we can get!