Interarea Route Summarization – Exclusive Explanation

Interarea route summarization in the OSPF must be manually configured on ABRs because OSPF does not support auto-summarization. We can perform internal routes summarization only on ABRs. When route summarization is configured on ABRs, a single type 3 LSA (summary LSA) describing the summary route are forwarded to the backbone area.

Multiple routes inside the area are summarized by the single LSA. A summarized route is generated if at least one subnet within the area comes down in the summary-address range. The summarized route metric is the same as the lowest cost of all subnets in the summary-address range. The ABR can only sum up routes that are within the areas connected to the ABR.

Figure 1 illustrates a multiarea OSPF topology. Figure 2 illustrates the routing tables of R3 before the route summarization is configured on R2.

Interarea Route Summarization
Interarea Route Summarization

Calculating the Summary Route

Summarizing different adjacent subnetworks into a single address and mask can calculate in three steps. The steps are the following:-

  1. List all networks in binary format. For example, the two networks of area 10, 192.168.0.0/24 and 192.168.1.0/24 are listed in binary format in figure 3 (1).
  2. Count the matching bits from left to right. The matching bits determine the mask for the summary route. As masked, the first 23 far left bits match. This results in the prefix/23 or subnet mask255.255.254.0. Figure 3(2) illustrates the counting of matching bits.
  3. Copy the matching bits from left to right and then add zero bits to remaining positions to determine the summarized network address as shown in figure 3(3). In this example, the matching bits with zeros at the result is a network address of 192.168.0.0/23. This summary-address summarizes two networks: 192.168.0.024, and 192.168.1.0/24. In the example the two networks matched out of three networks.
Interarea Route Summarization

Configuring Interarea Route Summarization

We can manually configure the route summarization on ABRs. The R2 is ABR for area 10 and area 0. In the example, R1 is configured to summarize the internal area 10 routes.

To configure route summarization manually on ABR, use the area <area-id> range <address> <mask> command on router configuration mode. This command instructs the ABR to summarize routes for a specific area before injecting them into a different area, via the backbone as type 3 summary LSAs. In our example the configuration on R2 is:

  • R2(config)#router ospf 10
  • R2(config-router)# area 10 range 192.168.0.0 255.255.254.0
  • R2(config-router)# do wr

In OSPFv3, the command is the same except for the IPv6 network address. The command syntax for OSPFv3 is area area-id range prefix/prefix-length.

Figure 4 displays the updated R3 routing table. You can examine that there is only one interarea route entry now available in the routing table for both routes. Though this example only reduced the routing table by one entry, summarization could be implemented, to sum up a lot of networks to reduce the size of routing tables extremely.

Interarea Route Summarization
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Asad Ijaz

NetworkUstad's lead networking architect with CCIE certification. Specializes in CCNA exam preparation and enterprise network design. Authored 2,800+ technical guides on Cisco systems, BGP routing, and network security protocols since 2018. Picture this: I'm not just someone who writes about tech; I'm a certified expert in the field. I proudly hold the titles of Cisco Certified Network Professional (CCNP) and Cisco Certified Network Associate (CCNA). So, when I talk about networking, I'm not just whistling in the dark; I know my stuff! My website is like a treasure trove of knowledge. You'll find a plethora of articles and tutorials covering a wide range of topics related to networking and cybersecurity. It's not just a website; it's a learning hub for anyone who's eager to dive into the world of bits, bytes, and secure connections. And here's a fun fact: I'm not a lone wolf in this journey. I'm a proud member and Editor of Team NetworkUstad. Together, we're on a mission to empower people with the knowledge they need to navigate the digital landscape safely and effectively. So, if you're ready to embark on a tech-savvy adventure, stick around with me, Asad Ijaz Khattak. We're going to unravel the mysteries of technology, one article at a time!"