Route summarization

Route summarization refers to the process of aggregating multiple routes of Internet-connected routers, thus saving space in the routing table and speeding up packet routing. An analogy would be on a U.S. interstate highway, where a single sign points in the direction of three to five major cities. As you draw nearer to your destination, the signs start separating for the distinct paths to each city. The same principle can be applied to route summarization.

Route summarization, also known as route aggregation, summarizes a group of routes into a single route advertisement. Route summarization can be used as a powerful tool in a networking environment. The demand for increased network capabilities has resulted from corporate expansions and mergers. The number of subnets and network addresses contained in routing table is rapidly increasing based on these expansions. This growth has had a negative impact on CPU resources, bandwidth, and memory used to maintain the routing tables. Therefore, route summarization was introduced as a way to reduce the size of network routing tables.

If configured properly, route summarization can reduce the latency associated with router hop, since the average speed for routing table lookup will be increased due to the reduced number of entries. The overhead for routing protocols can also be reduced since fewer routing entries are being advertised.

Another advantage of using route summarization in large, complex networks is that it can isolate topology changes from other routers. This can aid in improving the stability of the network by limiting the propagation of routing traffic after a network link goes down. For example, if a router only advertises a summary route to the next router hop, then it will not advertise any changes to specific subnets within the summarized range. This can significantly reduce any unnecessary routing updates following a topology change. Hence, increasing the speed of convergence and allowing for a more stable environment.

As an example of how summarization can be used as a powerful tool in a networking environment imagine a company that operates 150 accounting services in each of the 50 states and each accounting office has a router and frame relay link connected to its corporate office. Without route summarization, the routing table on any given router would have to maintain 150 routers times 50 states = 7,500 different networks. However, if route summarization is implemented, then each state would have a centralized site to connect it with all other offices. Since each router is summarized before being advertised to other states, then every router will only see its own subnets and 49 summarized entries representing other states. This would create less stress on the router’s CPU, memory, and bandwidth.

In order to determine the summary route on a router, you must first decide the number of highest-order bits that match in all addresses. See the following example which shows the process of calculating a summary route.

In the table below, Router A has the following networks in its routing table:

192.168.98.0
192.168.99.0
192.168.100.0
192.168.101.0
192.168.102.0
192.168.105.0

First of all, you must convert the addresses to binary format and align them in a list as shown in the table below.

Second, locate the bits where the common pattern of digits ends (those in red). Lastly, count the number of common bits. The summary route should be your lowest IP address, followed by a slash, followed by the number of common bits.

Summarized route is 192.168.96.0/20 (or 255.255.240.0)

As you can see, the first 20 bits of the IP address are the same. Hence, the best summary route can be advertised as 192.168.98.0/20.For summarization to work properly, multiple IP addresses must share the same highest-order bits and should only be implemented within classless routing protocols such as EIGRP, OSPF, RIP v.2, IS-IS for IP, and BGP.

In some cases, this feature may not be feasible. For example, in RIP v.1 is a classful routing protocol that automatically summarizes based on class when advertising across a major network boundary. Automatic route summarization can potentially cause problems if summarization occurs at more than one point in the network since the summarized routes may be in conflict. When this occurs, a router receives identical summary routes from different directions. This can lead to serious connectivity issues.