Calculating the Best Summary Route
Today’s post sets up the topic of how to find a “best” summary route. What does that mean, and how do you do it? Today’s post hits the highlights at least, and points to more info. Other later posts then give you some practice problems. Ask questions if you have them!
An Overview of Summary Routes
A summary route, sometimes called a manual summary route, is a route advertisement that lists a single route. The range of addresses that match that single route includes the same addresses in multiple other subnets in a router’s routing table. Instead of advertising those multiple routes for smaller subnets, the router instead advertises that one summary route. That summary route represents a superset of the range of addresses compared to the original routes.
The term manual summary route refers to the fact that the summary route requires a configuration command that the engineer must type on some router.
For the CCNA exam, you need to be ready to look at a set of routes in a router’s routing table, and find a larger subnet/mask combination that could be configured as a summary route. For instance, if a router had routes for subnets 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24, you could advertise a summary route for subnet 172.16.0.0/16, because it represents a superset of original three subnets, in that it includes all the addresses in the original three subnets, plus some others.
Many possible summary routes exist for a given set of routes. To determine the best summary route – in other words, the best subnet/mask combo to advertise as a summary – you need to think about the goals of a given network. However, for CCNA, best summary generally means the summary route that includes all the addresses in the original subnets, but is the smallest such subnet. For instance, with the earlier example, 172.16.0.0/16 includes all the addresses in the original three subnets, but the summary route includes many other addresses as well.
For example, consider the best summary route for those earlier three subnets: 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24. The earlier answer, 172.16.0.0/16, means “all addresses that begin 172.16”, which is far more addresses than the addresses in those three subnets. However, a summary route based on subnet 172.16.0.0/22 also includes all the addresses in those original three subnets, but not many others. In fact, 172.16.0.0/22 is the smallest single subnet that includes all the addresses in 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24.
You can use many methods to find the best summary route. If you have a method you already like, use it! Ignore what is here! But if you want to see another method, I’ve got a shorthand method of how to do it listed here. I’ll show the process in a few upcoming examples in some practice problems.
A Process to Find the Best Summary Route
Here’s the process, without a lot of embellishment, followed by 1 example. More examples to follow, once I show some practice problems.
Step 1 List all to-be-summarized subnet numbers in decimal, in order, lowest to highest, along with their matching subnet broadcast addresses
Step 2 Note the lowest Subnet ID and highest Subnet broadcast address, which define the range of addresses that must be inside the summary route
Step 3 Pick a starting point prefix length /P: the shortest prefix length mask of all the original subnets, minus 1
Step 4 Use the lowest numeric subnet ID from the original list of subnets, and the current prefix length, and calculate a new subnet ID and subnet broadcast address
A If the calculated range includes the entire range from step 2, you have found the best summary route
B If not, subtract 1 from the prefix length, and repeat step 4
An Example of the Process
For example, say you had the same problem as earlier in this post. You would have the following list of subnet IDs and Subnet broadcast addresses, with the lowest/highest noted in bold:
Table 1 – First Two Steps to Find Best Summary Route
|Subnet ID||Subnet Broadcast Address|
Next, at step 3, all three original subnets used prefix length /24, so the first prefix mask to try for the summary (step 3) will be one less, or /23. At step 4, you use the lowest original subnet ID, or 172.16.1.0, every pass through step 4, but each time, you use a progressively smaller prefix length. This first pass through step 4, with prefix mask /23, you calculate:
- Subnet ID 172.16.0.0/23, subnet broadcast 172.16.1.255
In this case, the range of addresses in the newly calculated subnet does not include the entire range from Table 1, so per step 4B, you repeat step 4, but with the prefix length 1 smaller, or /22. Passing through step 4 again:
- Subnet ID 172.16.0.0/22, subnet broadcast 172.16.3.255
In this case, the subnet (172.16.0.0/22) does include the entire range of addresses in the original subnets, so 172.16.0.0/22 is the best summary route.