Redes

CHAPTER 3

Route Summarization
Route summarization, or supernetting, is needed to reduce the number of routes that a router advertises to its neighbor. Remember that for every route you advertise, the size of your update grows. It has been said that if there were no route summarization, the Internet backbone would have collapsed from the sheer size of its own routing tables back in 1997!Routing updates, whether done with a distance vector or link-state protocol, grow with the number of routes you need to advertise. In simple terms, a router that needs to advertise ten routes needs ten specific lines in its update packet. The more routes you have to advertise, the bigger the packet. The bigger the packet, the more bandwidth the update takes, reducing the bandwidth available to transferdata. But with route summarization, you can advertise many routes with only one line in an update packet. This reduces the size of the update, allowing you more bandwidth for data transfer. Also, when a new data flow enters a router, the router must do a lookup in its routing table to determine which interface the traffic must be sent out. The larger the routing tables, the longer this takes,leading to more used router CPU cycles to perform the lookup. Therefore, a second reason for route summarization is that you want to minimize the amount of time and router CPU cycles that are used to route traffic.
NOTE: This example is a very simplified explanation of how routers send updates to each other. For a more in-depth description, I highly recommend you go out and read Jeff Doyle’s book RoutingTCP/IP, Volume I, 2nd edition, Cisco Press. This book has been around for many years and is considered by most to be the authority on how the different routing protocols work. If you are considering continuing on in your certification path to try and achieve the CCIE, you need to buy Doyle’s book — and memorize it; it’s that good.

Example for Understanding Route Summarization
Refer to Figure3-1 to assist you as you go through the following explanation of an example of route summarization.

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Example for Understanding Route Summarization

Figure 3-1

Four-City Network Without Route Summarization
172.16.64.0/24

172.16.65.0/24

172.16.66.0/24

Winnipeg

Vancouver

Seattle

172.16.67.0/24 172.16.68.0/24

172.16.69.0/24

Calgary

172.16.70.0/24 172.16.72.0/24172.16.71.0/24 172.16.73.0/24 172.16.79.0/24

Edmonton

172.16.78.0/24

172.16.74.0/24 172.16.75.0/24

172.16.77.0/24 172.16.76.0/24

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 routesummarization 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 are 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 24bits are common to all four networks. 172.16.64.0 = 10101100.00010000.01000000.00000000 172.16.65.0 = 10101100.00010000.01000001.00000000 172.16.66.0 = 10101100.00010000.01000010.00000000 172.16.67.0 = 10101100.00010000.01000011.00000000 Common bits: 10101100.00010000.010000xx You see that the first 22 bits of the four networks are common. Therefore, you can summarize the four routes by using asubnet 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 172.16.64.0/22

Example for Understanding Route Summarization

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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...