After a couple of weeks of packet tracer lab, are we back here to finish the remaining topics in CCNA prep. In this article we are going to discuss route summarization, the article most awaited and demanded by my regular readers. In a few simple words, route summarization is a method for consolidating a group of networks into a single network with the main objective of reducing the routing table size and complexity.

There are many millions of routes on the internet. If these routes all had to be stored individually, the internet would have come to a stop many years ago. Route summarization is also known as supernetting and was proposed in RFC 1338, which you can read online by clicking on the RFC. So make a cup of coffee or tea and concentrate for some time to understand how to use the route summarization technique in your network.


Route Summarization Prerequisites

In order to use route summarization on your network, you need to use a classless protocol, such as RIPv2, EIGRP, or OSPF. You also need to design your network in a hierarchical order, which will require careful planning and design. This means that you can’t randomly assign networks to various routers or LANs within your network.

Let’s take a look at the example shown in Fig. 1 (and see the table in Fig. 2):

From the above table, you can understand that the network id will be 172.16.8.0 with subnet mask 255.255.248.0 (subnet mask at /21).

You can only really work out a summary route by converting the IP address into binary. If you don’t convert this then you have no way of knowing if you are advertising the correct summary route, which will lead to problems on your network.

Routing protocols can summarize the addresses of several networks into one address, so we need to do proper manual summarization for specific information about that network, as we have done for the following example.

Can be called three different things:

  1. Route aggregation
  2. Route summarization
  3. Super-netting

Without route summarization, R2 has to have specific routes in its routing table to reach the three stub networks connected to R1. With route summarization, R2 only needs to have a summary route to reach all R1’s stub networks. R1 will locally determine which interface to use to route the packet. To determine a summary route, use the following procedure:

1. Identify the network with the highest and lowest IP addresses, 8.8.8.32/28 and 8.8.8.0/28 on R1.

2. Convert to binary the octet where the IPs start to differ.

• 8.8.8.32 -> 8.8.8.0010 0000

• 8.8.8.0 -> 8.8.8.0000 0000

3. Determine up to which point the two IPs match and convert to decimal, considering the remaining bits to the right equal zero, X=0:

• 8.8.8.00XX XXXX -> 8.8.8.0000 0000 -> 8.8.8.0

4. Count all bits where both IPs match to determine the subnet mask:

24 + 2 = 26

5. Create your summarized network, which is composed using the results from steps 3 and 4:

• 8.8.8.0/26

Let’s repeat the procedure for R2:

1. 8.8.9.48/30 and 8.8.8.0/26.

2. 8.8.0000 1001.0011 0000 and 8.8.0000 1000.0000 0000

3. 8.8.0000 100X.XXXX XXXX -> 8.8.0001 0000.0000 0000 -> 8.8.8.0

4. 16 + 7 = 23

5. 8.8.8.0/23

Network 8.8.8.0/23 can be used to summarize all four network behind R2.

Route summarization is very useful; however, there are cases where it might create some problems. For example, networks connected to R1 are summarized into 8.8.8.0/26. This summary also includes either network 8.8.8.24/29 or networks 8.8.8.24/30 and 8.8.8.28/30, which actually do not exist on this example.

So, an IP packet with destination address 8.8.8.25 may unnecessarily travel all the way to R1 to be discarded.

This situation is explained below. Good network planning should always be accomplished to help successful network summarization. Ideally, all networks should be continuous or as continuous as possible, like the ones shown in this example. It would be very difficult or impractical to summarize discontinuous networks, such as 10.1.55.0/24 and 200.26.58.28/30, for instance. The summarized network will include too many combinations, making it impractical to summarize them. The truncated configurations below allow static routing between R1 and R2 without summarization.

Route Summarization Strategies

The following example illustrates a method for summarizing a group of subnets:

10.3.4.0/24

10.3.5.0/24

10.3.6.0/24

10.3.7.0/24

1. Convert the addresses to binary.

2. Find the all the common bits from left to right.

3. Convert all the bits that are in common back to decimal, and leave all the rest of the bits to zero.

4. The number of bits that are in common will be their subnet mask:

10.3.4.0: 00001010 00000011 00000100 00000000

10.3.5.0: 00001010 00000011 00000101 00000000

10.3.6.0: 00001010 00000011 00000110 00000000

10.3.7.0: 00001010 00000011 00000111 00000000

The bits in red are the common bits. When the bits in red are converted back to decimal, the end result is 10.3.4.0. This is the summary subnet.

To calculate the mask, count all the bits in red. There are 22 bits. Therefore the summary route is:

10.3.4.0/22 or subnet mask 255.255.252.0

Manual Route Summarization

This procedure reduces the size of the routing table by aggregating a group of routes into a single route that is advertised to the other routers. As a result, routing packets is a speedier process and less processing power is consumed. This needs to be configured in order for it to occur; to manually configure a summary route with EIGRP (or RIP v2), use the interface subcommand ip summary-address

R1(config-if)#ip summary-address eigrp 1 10.2.0.0 255.255.0.0

R2(config-if)#ip summary-address eigrp 1 10.3.0.0 255.255.0.0

Here is how to manually calculate a summarized route:

172.1.4.0 /25

172.1.4.128 /25

172.1.5.0 /25

172.1.6.0 /24

172.1.7.0 /24

172.1. (Variation in third octet)

4.0 00000100.00000000

4.128 00000100.10000000

5.0 00000101.00000000

6.0 00000110.00000000

7.0 00000111.00000000

*Notice the 22nd place holder (the first six bits are same from the third octet, so 16+6 =22) is the last in common. Choose this place.

To find the IP subnet and subnet mask to summarize this route, you use the 22nd placeholder for the subnet mask (/22). And you keep the number 4 value in the 3rd octet because it is the value for the 22nd placeholder.

Let’s look at the example below:


Let’s try and summarize network 10.1.0.0 through 10.5.0.0:

First, list everything in binary.

To get the network address, follow and match the binary bits, starting on the left, and stop where the bits do not match from the example above.

Notice that the first octets are matched, the second octets have no matching bits, and the same is true of the third and last.

Therefore, the summary IP will be 10.0.0.0 = Network address.

Finally, to work out the summary subnets mask, we match the eight bits of the first octet (see above), which is the network, and five matching zeros in the second octet, which is the subnet.

255.248.0.0

How did we get the 248?

Remember the bits value = 128 64 32 16 8 4 2 1

1 1 1 1 1 0 0 0

You add the values of the five bits in the second octet from the left: 128+64+32+16+8 = 248

How did we get /13?

Count all the matching bits (see above) from the left up to the last matching bit to get your CIDR.

I hope reading this article and doing some hands-on practice will be enough for you to understand route summarization for the CCNA exam. If you can quickly work out the common bits then you should be able to answer the question quickly and accurately. I am waiting for your response about these articles, as readers’ feedback is always a valuable asset for any author. If you want to learn more about route summarization, then please get yourself a copy of Jeff Doyle’s book Routing TCP/IP, Vol. 1, 2nd Ed.

References

  • Guide to Cisco Certified Network Associate Certification by Todd Lamlee
  • Guide to Cisco Certified Network Associate by Richard Deal
  • Routing TCP/IP Vol. 1, 2nd Ed. by Jeff Doyle
  • CCNP Route Quick Reference by Denise Donohue, Ciscopress.com
  • Cisco Certified Internetwork Expert by Wendell Odom et al., Ciscopress.com
  • Cisco Certified Internetwork Expert Quick Reference by Brad Ellis, Ciscopress.com