A summary static route can be used to minimize the number of static routes in the routing table. Less routing table entries produce less CPU overhead and also provide better processing speed with faster routing table lookups. Using summary static routes we can also make management of a large number of static routes easy and less prone to errors. We use the summary static route when multiple networks point to the same exit interface or next hop. The Multiple static routes can be summarized into a single static route if:
- The destination networks are adjacent and can be summarized into a single network address.
- The multiple static routes all use the same exit interface or next-hop IP address.
In the figure, Router0 have configured four separate static routes to reach the 10.10.0.0/24 to 10.10.3.0/24 networks. All the networks are adjacent and also pointing to the same interface. So, we can configure a single route for all four routes. Classless Inter-Domain Routing (CIDR) is a form of route summarization. In the route summarization, we should do super netting of all subnet that required route summarization.
Summary Static Route Calculations
Let us calculate the summary static route for the routes mentioned in the above topology. First list all networks in binary format, the figure below illustrates all networks in binary formats.
Now count the common bits from left to right to determine the mask for the summary route. We highlight the matching bits in the figure below. This is the prefix for the summarized route: /22 or 255.255.252.0.
Copy the matching bits, and then add zero into the position of the remaining bits to determine the summarized network address. As shown in the figure below, we summarized networks into a single network address 10.10.0.0/22.
So, now we can summarize the four configured routes Router1 into only one summary route for all four networks. The summary route 10.10.0.0/22 has included all four networks.