Multiple static routes can be summarized into a single static route if:
- The destination networks are contiguous and can be summarized into a single network address.
- The multiple static routes all use the same exit interface or next-hop IP address.
Consider the example in Figure 1. All routers have connectivity using static routes.
Figure 2 displays the static routing table entries for R3. Notice that it has three static routes that can be summarized because they share the same two first octets.
Figure 3 displays the steps to summarize those three networks:
Step 1. Write out the networks to summarize in binary.
Step 2. To find the subnet mask for summarization, start with the far left bit, work to the right, finding all the bits that match consecutively until a column of bits that do not match is found, identifying the summary boundary.
Step 3. Count the number of far left matching bits; in our example, it is 22. This number identifies the subnet mask for the summarized route as /22 or 255.255.252.0.
Step 4. To find the network address for summarization, copy the matching 22 bits and add all 0 bits to the end to make 32 bits.
After the summary route is identified, replace the existing routes with the one summary route.
Figure 4 displays how the three existing routes are removed and then the new summary static route is configured.
Figure 5 confirms that the summary static route in is the routing table of R3.