In small inter-networking environments, the routers routing tables might only have a few dozens of routes. The larger the network, the larger the number of routes are. When the number of the routes in the router’s routing tables grows bigger and bigger, they consume more memory in the router and take more time in searching the routing tables when they need to route a packet. Route summarization is a technique that helps to reduce the size of the routing tables.
Mostly the routing protocols such as EIGRP, OSPF, and Rip support VLSM (variable-length subnet masks). When the network uses route summarization, the routing protocols must support VLSM. VLSM means that, in a single Class A, B, or C network, more than one subnet mask value is used. If the same mask is used throughout the same network, many subnets might have far more IP addresses than will ever be used. VLSM allows some subnets to be smaller and some to be larger, which reduces wasted IP addresses.
For example in a point-to-point serial links connection, you only need 2 IP addresses one IP for each end of the serial links and most networks use the subnet mask 255.255.255.252. It would be wasting the numbers of IP addresses when you use the default single class C subnet mask (255.255.255.0) if you only need 2 IP addresses. On LAN subnets the network uses a mask that allows larger subnets, such as 255.255.255.0. See also how to calculate subnet mask.
Network administrators know that route summarization is used to reduce the size of the routing tables in the network. Route summarization causes some number of more-specific routes to be replaced with a single route that includes all the IP addresses covered by the subnets in the original routes. Route summarization works much better when the network was designed with route summarization in mind. The following figure shows a corporate network for which route summarization was planned.
Calculate the Route Summarization
In this example, the network administrator planned all the subnets numbers relative to the goal of using the route summarization. All subnets off the main site (Jakarta), including the WAN links start with 10.101. All LAN subnets off Sydney start with 10.105, and all LAN subnets off Chicago start with 10.100.
Based on the above diagram, Jakarta routing table originally (without summarization) shows four routes to subnets that begin with 10.105, all pointing out its serial s0/0 interface to Sydney. You can do similarly for the Chicago subnets to add to Jakarta routing tables – four routes to subnets that begin with 10.100, all pointing out its serial 0/1 interface to Chicago. All of the above subnets are using the subnet mask 255.255.255.0 in all cases except WAN serial links between Jakarta, Sydney, and Chicago. Now how can we make route summarization?
Let’s have a look at the below table that shows all the subnets of Sydney network. First of all, you need to convert all subnets into their binary form as shown. Since all the subnet use subnet mask 255.255.255.0 – all the last (the 4th octet) octet will be ignored in this calculation. We see that all the bits in the first and the second octet have the same bits formation. And we also see that all the third octets have different bits only in the last 2 bits (green mark), while the first 6 bits are the same.
Now it’s easy to mask the same bits with the ‘1’ as shown, and we can convert the first and the second octets to decimal 255, while the third octet is converted to 252. So the subnet mask result is 255.255.252.0, and now we can summarize all the subnets to become 10.105.4.0 with subnet mask 255.255.252.0 (22 bits ‘1’) or we can write it down in this notation 10.105.4.0/22. 22 here is the total number of “1” in the mask.
What about the routing table for all subnets off the Chicago router? We know that all the subnets in Chicago starts with 10.100. We can do similar way in calculating the subnet summarization using the above figure, and you will find the same subnet mask 255.255.252.0. The route summarization can be written as 10.100.4.0 /22 for Chicago router.
To be continued
See also:
- General routing guide
- Static route – the concept
- Guidelines in setting up the router
- Router password – configuration
- Backup hard disk – how to
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