Refer to Figure 3-1 to assist you as you go through the following explanation of an example of route summarization.
Figure 3-1. Four-City Network Without Route Summarization
As you can see from Figure 3-1, Winnipeg, Calgary, and Edmonton each have to advertise internal networks to the main router located in Vancouver. Without route summarization, Vancouver would have to advertise 16 networks to Seattle. You want to use route summarization to reduce the burden on this upstream router.
Step 1: Summarize Winnipeg's Routes
To do this, you need to look at the routes in binary to see if there any specific bit patterns that you can use to your advantage. What you are looking for are common bits on the network side of the addresses. Because all of these networks are /24 networks, you want to see which of the first 24 bits are common to all four networks.
You see that the first 22 bits of the four networks are common. Therefore, you can summarize the four routes by using a subnet mask that reflects that the first 22 bits are common. This is a /22 mask, or 255.255.252.0. You are left with the summarized address of
This address, when sent to the upstream Vancouver router, will tell Vancouver: "If you have any packets that are addressed to networks that have the first 22 bits in the pattern of 10101100.00010000.010000xx.xxxxxxxx, then send them to me here in Winnipeg.
By sending one route to Vancouver with this supernetted subnet mask, you have advertised four routes in one line, instead of using four lines. Much more efficient!
Step 2: Summarize Calgary's Routes
For Calgary, you do the same thing that you did for Winnipeglook for common bit patterns in the routes:
Once again the first 22 bits are common. The summarized route is therefore:
Step 3: Summarize Edmonton's Routes
For Edmonton, you do the same thing that we did for Winnipeg and Calgarylook for common bit patterns in the routes:
For Edmonton, the first 21 bits are common. The summarized route is therefore:
Figure 3-2 shows what the network looks like, with Winnipeg, Calgary, and Edmonton sending their summarized routes to Vancouver.
Figure 3-2. Four-City Network with Edge Cities Summarizing Routes
Step 4: Summarize Vancouver's Routes
Yes, you can summarize Vancouver's routes to Seattle. You continue in the same format as before. Take the routes that Winnipeg, Calgary and Edmonton sent to Vancouver and look for common bit patterns:
Because there are 20 bits that are common, you can create one summary route for Vancouver to send to Seattle:
Vancouver has now told Seattle that in one line of a routing update, 16 different networks are being advertised. This is much more efficient than sending 16 lines in a routing update to be processed.
Figure 3-3 shows what the routing updates would look like with route summarization taking place.
Figure 3-3. Four-City Network with Complete Route Summarization