Route Summarization and Auto Summarization Explained

This tutorial explains route summarization and auto summarization features in detail through examples. Learn how route summarization and auto summarization features work in networking.

What is the route summarization?

Instead of advertising individual contiguous routes, routing protocols generally prefer to advertise a single summarized route that represents all contiguous routes. This feature is known as the root summarization.

Technically, route summarization is a feature that takes a bunch of contiguous routes from the routing table and advertises them as a single summarized or aggregated route. This feature works only if two or more routes are contiguous. If routes are not contiguous, this feature will not work.

A network is considered contiguous when all of its hosts can access other hosts of the same network without going outside of the network. To access any other host of the network, if a host has to go outside the network then the network is considered as a discontiguous network.

Auto summarization

If a routing protocol automatically creates a summary route, it is known as the auto-summarization. In simple words, auto summarization is a state in which, under certain conditions, a routing protocol automatically uses the route summarization feature.

One such a condition is when the router sits between classful networks with some interface in one Class A, B, or C network and other interfaces in another Class A, B, or C network. In such a situation, the router automatically summarizes contiguous routes as a single classful network address.

The following image shows an example of this situation.

In this network, three routers R0, R1, and R2 are connected. R0's serial interface S0/0/0 is connected to R1's serial interface S0/0/0 and R1's serial interface S0/0/1 is connected to R2's serial interface S0/0/0. Networks 192.168.1.0/30 and 192.168.1.4/30 are used to connect serial interfaces, respectively.

Four subnets of the network 10.0.0.0 are connected to the left side of the router R0. How router R0 will advertise these routes on the right side from the serial interface depends on whether the route summarization feature is enabled or not.

If the route summarization feature is disabled, the router R0 will advertise these routes as they are. Router R1 will receive these advertised routes on its serial interface and will add them to its routing table.

The following image shows how this process works.

But if the route summarization feature is enabled, instead of advertising four contiguous routes separately, router R0 will advertise a single summarized route from the serial interface. Router R1 will also receive a single summarized route and will add that route to its routing table.

The following image shows this process.

Advantages of route summarization

Route summarization provides the following advantages.

It reduces the size of routing updates. As you can see in our example, when router R0 advertised routes without the summarization, it had to advertise four routes. But when it advertised with the summarization, it had to advertise only one route. Small size routing updates consume less bandwidth.

It reduces the size of the routing table. In our example, router R1 had to add four routes to its routing table when routes were not summarized. But when routes were summarized, it had to add only one route to its routing table. Small size routing tables require less CPU power and memory in processing.

It keeps routing tables stable by hiding flapping routes. For example, suppose that one subnet 10.2.0.0/24 is flapping. This flapping route affects the connected router R0. But this flapping route will affect any other router (in our example, R1) that accesses this route through R0 depends on how R0 advertises this route. If this route is advertised as an individual route, then it will affect other routers as well.

But if this route is advertised with other routes as a summarized route, it will not affect other routers. Other routers will affect only when all routes fail, causing the router (R0) to perform the summarization again and to stop advertising the summarized route.

Disadvantages of route summarization

Route summarization hides the layout of the network. This causes routers to make wrong decisions. For instance, suppose the network10.2.0.0/24 is down. If the summarization feature is disabled, R0 will immediately broadcast this information to other routers and they will know that the network 10.2.0.0/24 is down. But if the summarization feature is enabled, R0 will not be able to pass this information to other routers.

If the summarization feature is enabled, R0 will broadcast a summarized route that contains all available contiguous routes. No matter whether this summarized route contains the routes which are down or even not available. Due to this, R1 will never know that the network 10.2.0.0/24 is down. From R1's perspective, since R0 is advertising a route 10.0.0.0/8, all addresses from 10.0.0.0 through 10.255.255.255 are available via R0. Obviously, this not true. This wrong assumption causes a lot of problems. For example, even the network 10.2.0.0/24 is down, R1 still thinks that this network is available via R0 and it will keep sending traffic of 10.2.0.0/24 to the R0.

Route summarization and discontiguous subnets

If the network contains discontiguous subnets, route summarization should not be used. To understand how the route summarization causes trouble if the network contains discontiguous subnets, let's expand our example internetwork.

To replicate this situation, add a few subnets of 10.0.0.0/8 to the right side of the router R2 and enable route summarization of both routers R0 and R2.

The following image shows this expended version of the internetwork.

Since route summarization is enabled on both routers (R0 and R1) and both routers contain the subnets of the same network (10.0.0.0), they will both advertise the same summarized route (10.0.0.0/8) to router R1.

Router R1 receives the same summarized route on its serial interfaces S0/0/0 and S0/0/1. Since received routes are the same, R1 adds both routes to its routing table as a single route through a special feature known as the load balancing.

If multiple routes for a single destination are merged through the load balancing feature, based on network conditions, the router can use any route from them to forward the traffic.

When R1 receives data packets for the network 10.0.0.0/8, depending on network condition, it can use either serial 0/0/0 or serial 0/0/1 to forward them. This approach does not work properly. By using this approach, sometimes R1 may forward packets to the correct destination and sometimes not.

For example, suppose R1 receives a packet for the destination network 10.5.0.0/24 and it forwards that packet to the 192.168.1.1 (R0), then it will be incorrect. Because R0 has no route for the destination network 10.5.0.0/24.

Same way, if R1 receives a packet for the destination network 10.1.0.0/24 and it forwards that packet to 192.168.1.6, then it will be also incorrect. Because R2 has no route for the destination network 10.1.0.0/24.

Packets forwarded to an incorrect destination are destroyed.

Enabling/ disabling automatic route summarization

Different routing protocols use route summarization feature differently. For example, RIPv2 and EIGRP not only support the route summarization feature but also automatically summarize contiguous networks across the network class boundaries. On the other hand, OSPF does not summarize any type of routing information.

If the routing protocol supports the automatic route summarization feature, depending on your network layout and requirement, you can enable or disable this feature.

To enable automatic route summarization, use the 'auto-summary' command from the router sub-config mode.

The following image shows how to enable automatic route summarization on RIPv2.

To disable automatic route summarization, use the 'no auto-summary' command from the router sub-config mode.

The following image shows how to disable automatic route summarization on EIGRP.

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