Route Summarization Advantages and Disadvantages

Routers learn routes from route advertisements. Routers advertise the routing information they have in their routing tables. Route summarization allows routers to advertise summarized routes.

Subnetting breaks IP subnets into smaller IP subnets. Subnetting provides many benefits, but it also creates overhead. For example, if you break an IP subnet into eight IP subnets, routers must learn and broadcast eight IP subnets instead of a single IP subnet.

Route summarization solves this issue. It allows routers to advertise contiguous networks as a single summarized or aggregated route. They can use it to summarize subnetted routes back to the class boundary.

For example, the following image shows a network that uses subnetted networks. All subnetted routes are on the left side of the production router. The production router can advertise these routes to the right side in two ways: individually and collectively. It uses the route summarization feature to advertise them collectively.

The following figure shows an example of route summarization.

Advantages of Route Summarization

Route summarization provides the following advantages.

• It reduces the size of routing tables. Routers store only summarized routes in their routing tables, not subnetted routes.
• Routers need less memory and processing power as they process fewer routes.
• It requires less bandwidth as routers advertise fewer routing updates.
• It hides network problems such as a flapping route.
• It provides a better overview of the network.
• It reduces resource usage, such as Memory and CPU.
• It reduces the time required to rebuild the routing tables.

Disadvantages of route summarization

The main disadvantages of route summarization are the following.

• It adds complexity to the network.
• Summarized routes are confusing. You can decode them only if you know how subnetting works.
• Summarized routes may contain errors. For example, they may advertise a down subnet as an up subnet.

Route summarization and flapping routes

A flapping route is one that goes up and down, up and down. Loose connections and short circuits are two common causes of flapping. A flapping route affects the connected router and any other router that knows about it. Every time it goes up or down, the other routers have to update their routing tables.

However, if routers know only the summarized route, they will not be affected by the flapping route. They will affect only when all four subnets fail, causing the router performing the summarization to stop advertising the summarized route.

Hiding a flapping route has both advantages and disadvantages. On the positive side, it provides network stability. Routers update their routing tables only when all routes for a summarized route fail. On the downside, it hides the full picture of the network.

It can cause routers to make bad assumptions. Let us understand it through the preceding example. Suppose the network 192.168.1.128/26 is down. Routers on the right side of the production router will still receive updates concerning the summarized route (192.168.1.0/24). From their perspective, since the production router is still advertising this route, all addresses from 192.168.1.0 through 192.168.1.255 must be available. Obviously, this is not true, so other routers will still send traffic to 192.168.1.128/26 because they think it’s still reachable.

Conclusion

Route summarization is a powerful technique that optimizes network performance by reducing routing table sizes, lowering resource usage, and improving stability. However, it also introduces some complexity and may obscure specific network issues, such as individual subnet failures. To maximize the benefits of route summarization, you should understand its advantages and potential drawbacks and apply it thoughtfully in our network designs.