Determining the Summary EIGRP Route

Figure 1 shows the two manual summary routes that are configured on R3. These summary routes are sent out of the Serial 0/0/0 and Serial 0/0/1 interfaces to R3’s EIGRP neighbors.

To determine the summary of these three networks, the same method is used to determine summary static routes, as shown in Figure 2:

Step 1. Write out the networks to be summarized in binary.

Step 2. To find the subnet mask for summarization, start with the far left bit.

Step 3. Working from left to right, find all the bits that match consecutively.

Step 4. When there is a column of bits that do not match, stop. This is the summary boundary.

Step 5. Count the number of far left matching bits, which in this example is 22. This number is used to determine the subnet mask for the summarized route: /22 or

Step 6. To find the network address for summarization, copy the matching 22 bits and add all 0 bits to the end to make 32 bits.

The result is the summary network address and mask for

Configure EIGRP Manual Summarization

To establish EIGRP manual summarization on a specific EIGRP interface, use the following interface configuration mode command:

Router(config-if)# ip summary-address eigrp as-number network-address subnet-mask

Figure 2 shows the configuration to propagate a manual summary route on R3’s Serial 0/0/0 interface. Because R3 has two EIGRP neighbors, the EIGRP manual summarization must be configured on both Serial 0/0/0 and Serial 0/0/1.

Use the Syntax Checker in Figure 3 to configure the same manual summary route on R3’s Serial 0/0/1 interface.