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.
We use subnetting to break IP subnets into small IP subnets. Subnetting provides many benefits, but it also creates overhead. For example, if we 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 the routing table instead of subnetted.
- 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 decreases the use of resources such as Memory and CPU.
- It decreases the required time to rebuild the routing tables.
This tutorial is the eleventh chapter of the tutorial series. Other chapters of this series are the following.
Chapter 01 Introduction to Subnetting
Chapter 02 Network Address Basic Concepts Explained with Examples
Chapter 03 The Subnet Mask and Slash Notation
Chapter 04 Convert Decimal IP address in Binary and Binary in Decimal
Chapter 05 Basic Subnetting in Computer Networks Explained
Chapter 06 Subnetting Tutorial - Subnetting Explained with Examples
Chapter 07 Subnetting Tricks Subnetting Made Easy with Examples
Chapter 08 FLSM Subnetting and VLSM Subnetting
Chapter 09 VLSM Subnetting Explained with Examples
Chapter 10 VLSM Subnetting Examples and Calculation Explained
Chapter 12 Supernetting Tutorial: - Supernetting Explained with Examples
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 a route that is going up and down, up and down. Loose connections and short circuits are two common reasons that cause 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 incorporate the change into 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 stability in the network. Routers change their routing table only when all routes of a summarized route fail. On the negative side, it hides the complete 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, and thus other routers will still send traffic to 192.168.1.128/26 since they think it’s still reachable.
