Summarization and How to Configure Summarization

Summarization, put simply, enables a given routing protocol to minimize IP routing tables by taking steps to advertise a smaller or lesser IP route destination for a large set of subnets or networks. IP routing entries consume bandwidth of expensive links between different geographic locations, take CPU cycles on routers, and, most importantly, require memory.

To give network designers the ability to manage large networks, summarization is important for limiting or reducing IP routing tables. The most important consideration to make when summarizing any IP address space is to ensure a hierarchical design.

In a hierarchical design, IP address space is configured across any given router so that it can be easily summarized. To illustrate the capabilities of summarization consider the following IP address ranges in Table 1-3.

Table 1-3. IP Address Range

IP Subnet Binary Last Third Octet
131.108.1.0/24 0000 0001
131.108.2.0/24 0000 0010
131.108.3.0/24 0000 0011
131.108.4.0/24 0000 0100
131.108.5.0/24 0000 0101
131.108.6.0/24 0000 0110
131.107.7.0/24 0000 0111

A router would normally advertise each of the seven IP address ranges, from 131.108.1–7, as seven different IP route entries.

The binary examination of the subnets 1 to 7 in Table 1-3 displays that the first five bits (shaded) are unchanged. The most important fact is that these seven networks are contiguous or in a range that you can easily summarize. Because the high-order bits are common in Table 1-3 (0000 0) and all seven routes are contiguous (binary 001 to 111), you can perform summarization. Because the first five bits are the same, you can apply the mask 248 (11111 000) on the third octet and send an advertisement encompassing all seven routes. Before looking at how to complete this summarization using RIP, EIGRP, or OSPF, the following is a list of benefits when using summarization:

  • Reduces routing table sizes
  • Allows for network growth
  • Simplifies routing algorithm recalculation when changes occur
  • Reduces requirements for memory and CPU usage on routers significantly

The alternatives to network summarization are not easy to accomplish, and this includes renumbering an IP network or using secondary addressing on Cisco routers, which is not an ideal solution for management purposes and also provides extra overhead on a router. Also, it is important to understand that if a range of addresses is not contiguous (that is, they do not start from a range that can be easily summarized, such as the range of addresses 131.108.1.0/24 and 131.108.10.0/24), summarization is impossible. You could still summarize the first seven networks, for example, but they might reside in other parts of your network and cause IP routing problems. The best practice is to assign a group of addresses to a geographic area so that the distribution layer of any network enables summarization to be relatively easy to complete.

Depending on the routing protocols in use, summarization may be enabled by default. Automatic summarization simply announces a Class A network with an 8-bit mask, 255.0.0.0, Class B with 16-bit mask, and a Class C mask with a 24-bit mask, 255.255.255.0. With RIPv2, automatic summarization occurs. In other words, you must disable automatic summarization to allow the more specific routes to be advertised; otherwise a default mask is assumed.

To disable automatic summaries with RIPv2, use the following command:


   router rip
 
   version 2
 
   no auto-summary

The command no auto-summary disables automatic summaries and allows subnets to be advertised.

EIGRP also applies automatic summaries but it also enables the manual configuration of summary addresses. The following example shows you how to summarize the networks in Table 1-3 using EIGRP.

To configure summarization with EIGRP, you must first disable automatic summarization with the following command:


   router eigrp 1
 
   no auto-summary

Then, you apply the manual summarization on the interface to which you want to send the advertised summary. Example 1-1 displays the command you use to summarize the seven networks in Table 1-3.

Example 1-1. Summary with EIGRP

interface serial 0
 ip summary-address eigrp 1 131.108.1.0 255.255.248.0

Example 1-1 applies a summary on the serial interface. Also note that the EIGRP autonomous system number is 1, matching the configuration on the router because you can have more than one EIGRP process running. The actual summary is 131.108.1.0 255.255.248.0, which replaces the seven individual routers numbered 131.108.1-7.0/24 with one simple route.

OSPF allows summarization manually under the OSPF process ID. Now look at how to configure the seven networks in Table 1-3 with an OSPF summary. You use the following command in OSPF to summarize internal OSPF routes:


   area 
   area-id 
   range 
   address mask

Example 1-2 displays the configuration required to summarize the seven networks in Table 1-3. Assume the area-id for now is 1.

Example 1-2. OSPF summary

router ospf 1
area 1 range 131.108.1.0 255.255.248.0

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