Route summarization -- also known as route aggregation -- is a method to minimize the number of routing tables in an IP network. It consolidates selected multiple routes into a single route advertisement. This differentiates it from flat routing, in which every routing table carries a unique entry for each route. In basic terms, a route advertisement contains information that devices can use to communicate across an IP network.
Route summarization advertises a bunch of contiguous routes from the routing table in the form of a single summarized or aggregated route. For this feature to work, two or more routes must be contiguous, i.e., adjacent or adjoining.
In an enterprise network, subnetting is the process of lengthening the mask to create multiple smaller subnetworks, or subnets. Route summarization is the opposite process. It involves shortening the mask to include several smaller networks into one larger network address. This is why it is also known as supernetting.
As the network size increases, the number of individual networks listed in the IP route table also increases, as does packet size. Routers cannot effectively handle a large number of subnetworks, which leads to slowdowns, packet losses and even crashes. That's why it's important to reduce the number of entries in the route table, which is what route summarization accomplishes.
With route summarization, many routes are advertised with just one line in an update packet, which not only reduces the packet size, but also allows more bandwidth for data transfer.
If a router needs to advertise 50 routes, it will need 50 specific lines in its update packet. As these routes increase, the number of lines required also increases, expanding packet size and the amount of bandwidth used. That means there will be less bandwidth available for actual data transfer.
Route summarization enables multiple routes to be advertised with only one line in an update packet, reducing the packet size and leaving more bandwidth for data transfer.
Also, each time a new data flow enters a router, it must identify which interface the traffic must be sent out to. For this, it must perform a lookup in its routing table. This process takes longer for large routing tables and requires more router central processing unit (CPU) cycles to route traffic.
Route summarization can eliminate this problem by minimizing both the time required to perform lookup and reducing the number of CPU cycles.
Route aggregation offers several advantages, including the following:
There are two main disadvantages of route aggregation:
To avoid suboptimal or incorrect routing and to prevent routers from inaccurately advertising networks or duplicating other routers' advertisements, it's important to design networks with summarization in mind. Advance planning and leaving room for future network growth can help with the design of a scalable network that supports route summarization.
Depending on the routing protocol, network layout and requirements, the route summarization feature can be enabled or disabled.
Different routing protocols use this feature differently. For example, to implement route summarization in IPv4, Classless Inter-Domain Routing must be used. All IP addresses in the route advertisement must share identical high-order bits. The length of the prefix must not exceed 32 bits.
Routing Information Protocol (RIP) and Enhanced Interior Gateway Routing Protocol both support route summarization. They also summarize all the contiguous networks across the network class boundaries automatically. On the other hand, Open Shortest Path First, a link-state routing protocol, does not summarize any type of routing information.
To enable automatic route summarization on RIPv2, for example, the auto-summary command must be used from the router's sub-config mode.
To disable automatic route summarization, the no auto-summary command must be used from the same mode.
21 Oct 2021