Route Summarization

IP route summarization is a mechanism used to design networks that are more flexible and perform better. Although some routing protocols summarize only at the boundaries of major network numbers , others support route summarization (aggregation) at any bit boundary. Variable-length subnet masks enable routing protocols to summarize on bit boundaries. This aggregation process can work hand-in-hand with VLSM to build hierarchically structured networked environments. The following are some of the many advantages to summarizing addresses into a hierarchy:

• Reduces the amount of information stored in routing tables

• Allocates an existing pool of addresses more economically

• Makes the routing process more efficient

• Lessens the load on router processor and memory resources

• Lowers the network convergence time

• Isolates topology changes

• Facilitates monitoring, reporting, and troubleshooting

If no method existed for route summarization, every router would need to have a route to every subnet in the network environment. With summarization, routers can condense some groups of routes down to a single link advertisement, resulting in a reduction in both the resource load on the router and the overall network complexity. As the size of the network gets bigger, the more important the function of route summarization becomes.

The reduction in route propagation and routing information overhead can be significant. Take our previous sample network of 172.16.1.0 /25. Without summarization, each router in a large enterprise network of 500 subnets (2 ⁹ = 512 subnets with 2 ⁷ “ 2 = 126 hosts each) would need to know about 500 routes. With route summarization, you can quickly reduce the size of the routing tables by almost 75%. If the 172.16.0.0 Class B network used 7 bits of subnet address space (/23) instead of 9 bits (/25), the original 500 subnets could be broken up into four major subnetworks of 125. Each router would still need to know all the routes for each subnet in its network number. However, that number would be reduced to 125 routers plus one additional route for each of the other three major networks, for a total of only 128 routes. This process of collapsing many subnet routes into a single network route is a fundamental goal of route summarization. This can be seen using the 172.16.0.0 /25 subnet in Figure 3.4.

Figure 3.4. Routers share the first 16 high-order bits and summarize on network 172.16.0.0 /16.

In Figure 3.4, RouterA advertises that it can route from the 172.16.0.0 /25 subnets to the 172.16.0.0 /16 network via RouterB. Instead of sending four routing update entries, RouterA can summarize these addresses into just one network number. Another advantage is that if one of the interfaces on a downstream router is flapping, the summarized route 172.16.0.0 /16 does not change and RouterB is unaffected.

Here is one more common summarization scenario with EIGRP or OSPF as a routing protocol. Suppose an organization is using the familiar 172.16.0.0 /24 addressing scheme with 8 bits for subnetting. You must first determine whether a range of addresses can even be summarized. Table 3.11 shows a portion of the third octet of subnet addresses that can be summarized.

Table 3.11. IP Subnet Address Range of the Third Octet for Summarizing 172.16.0.0 /24

Subnet	Mask	Subnetwork Binary
172.16.1.0	255.255.255.0 (/24)	.0000 0001
172.16.2.0	255.255.255.0 (/24)	.0000 0010
172.16.3.0	255.255.255.0 (/24)	.0000 0011
172.16.4.0	255.255.255.0 (/24)	.0000 0100
172.16.5.0	255.255.255.0 (/24)	.0000 0101
172.16.6.0	255.255.255.0 (/24)	.0000 0110
172.16.7.0	255.255.255.0 (/24)	.0000 0111
172.16.8.0	255.255.255.0 (/24)	.0000 1000
172.16.9.0	255.255.255.0 (/24)	.0000 1001
172.16.10.0	255.255.255.0 (/24)	.0000 1010
172.16.11.0	255.255.255.0 (/24)	.0000 1011
172.16.12.0	255.255.255.0 (/24)	.0000 1100
172.16.13.0	255.255.255.0 (/24)	.0000 1101
172.16.14.0	255.255.255.0 (/24)	.0000 1110
172.16.15.0	255.255.255.0 (/24)	.00001111

Look at the binary representation of the subnets 1 “16. A common thread appears: The first four high-order bits are the same. The binary representation of those four common high-order bits is 240 (11110000), so you can summarize all these subnets to the subnet mask of 255.255.255.240 in an OSPF or EIGRP network. An EIGRP-loaded router, for example, would need to send only 1 update for the range of 172.16.0.0 “172.16.15.0 as opposed to 16 separate updates, as shown in Listing 3.2.

Listing 3.2 A Sample EIGRP Summarization Configuration

 RouterA(config)# router eigrp 10 RouterA(config-router)# no auto-summary RouterA(config)# interface serial 0/1  RouterA(config-if)# ip summary-address eigrp 10 172.16.1.0 255.255.255.240  

When a router is choosing an optimal route, the longest prefix is used to route the packet. A prefix is the slash symbol followed by a number that represents the total number of bits used for the network and subnet portion of the IP address, for example, /25.

With EIGRP, you need to include a no auto-summary entry because EIGRP automatically summarizes IP subnetworks. OSPF does not need this command because it does not auto-summarize.

To properly implement summarization in a network, the multiple addresses must share similar high-order bits. In Figure 3.4, the routers all share the first 16 high-order bits and summarize on 172.16.0.0 /16, and in Table 3.11, the routers all share the first 28 high-order bits and summarize on 172.16.1.0 /28. In addition, the routing protocol being used must transport the subnet mask information (called the prefix length ) as well as a 32-bit address.