Route Summarization and Variable-Length Subnet Masks 223

Foundation Topics

Route Summarization and Variable-Length Subnet Masks

Small networks might have only a few dozen routes in their routers’ routing tables. The larger the network, the larger the number of routes. In fact, Internet routers have more than 100,000 routes in some cases.

When larger IP networks are created, the routing table might become too large. As routing tables grow, they consume more memory in a router. Also, each router can take more time to route a packet, because the router has to match a route in the routing table, and searching a larger table generally takes more time. And with a large routing table, it takes more time to troubleshoot problems, because the engineers working on the network need to sift through more information.

Therefore, reducing the size of the IP routing tables has some advantages. Route summarization reduces the size of routing tables while maintaining routes to all the destinations in the network. Summarization also improves convergence time, because the router that summarizes the route no longer has to announce any changes to the status of the individual subnets. By advertising only that the whole summary route is either up or down, the routers that have the summary route do not have to reconverge every time one of the component subnets goes up or down.

The routing protocol must support variable-length subnet masks (VLSMs) when the network uses route summarization.

VLSM means that, in a single Class A, B, or C network, more than one subnet mask value is used. If the same mask is used throughout the same network, many subnets might have far more IP addresses than will ever be used. VLSM allows some subnets to be smaller and some to be larger, which reduces wasted IP addresses. This also allows the network to grow, without the need to reserve another registered network number with the Network Information Center (NIC).

VLSM can save a significant number of IP addresses, even just by using two different masks. For instance, many networks use a mask of 255.255.255.252 on point-to-point serial links because it allows only two valid IP addresses (you need only two IP addresses on a point-to- point serial link). Then, on LAN subnets, the network uses a mask that allows larger subnets, such as 255.255.255.0. The network uses VLSM because two different masks are used in the same Class A, B, or C network.

224 Chapter 7: Advanced Routing Protocol Topics

This section starts by describing route summarization concepts, followed by VLSM. At the end of this section, you will read about some tips for finding the answers to summarization questions on the ICND exam.

Route Summarization Concepts

Engineers use route summarization to reduce the size of the routing tables in the network. Route summarization causes some number of more-specific routes to be replaced with a single route that includes all the IP addresses covered by the subnets in the original routes.

Summary routes, which replace multiple routes, must be configured by a network engineer. Although the configuration command does not look exactly like a static route command, the same basic information is configured—but now the routing protocol advertises just the summary route, as opposed to the original routes.

Route summarization works much better when the network was designed with route summarization in mind. Figure 7-1 shows a network for which route summarization was planned.

In this network, the engineer planned his choices of subnet numbers relative to his goal of using route summarization. All subnets off the main site (Albuquerque), including WAN links, start with 10.1. All LAN subnets off Yosemite start with 10.2, and likewise, all LAN subnets off Seville start with 10.3.

Albuquerque’s routing table, without summarization, shows four routes to subnets that begin with 10.2, all pointing out its serial 0/0 interface to Yosemite. Similarly, Albuquerque shows four routes to subnets that begin with 10.3, all pointing out its serial 0/1 interface to Seville. Therefore, summarization can be used to make Albuquerque replace the four routes to 10.2 subnets with one route, and likewise for the four routes to 10.3 subnets. First, examine the routing table on Albuquerque shown in Example 7-1, with EIGRP as the routing protocol.

226 Chapter 7: Advanced Routing Protocol Topics

Example 7-2 Albuquerque Routing Table After Route Summarization (Continued)

10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks

D10.2.0.0/16 [90/2172416] via 10.1.4.2, 00:05:59, Serial0/0

D10.3.0.0/16 [90/2172416] via 10.1.6.3, 00:05:40, Serial0/1 C 10.1.1.0/24 is directly connected, Ethernet0/0

C 10.1.6.0/24 is directly connected, Serial0/1 C 10.1.4.0/24 is directly connected, Serial0/0

Example 7-3 Yosemite Configuration and Routing Table After Route Summarization

Yosemite#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Yosemite(config)#interface serial 0/0

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

Yosemite(config-if)#^Z

P - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks

D10.2.0.0/16 is a summary, 00:04:57, Null0

D10.3.0.0/16 [90/2684416] via 10.1.4.1, 00:04:30, Serial0/0 C 10.2.1.0/24 is directly connected, FastEthernet0/0

D10.1.1.0/24 [90/2195456] via 10.1.4.1, 00:04:52, Serial0/0 C 10.2.2.0/24 is directly connected, Loopback2

C 10.2.3.0/24 is directly connected, Loopback3 C 10.2.4.0/24 is directly connected, Loopback4

D10.1.6.0/24 [90/2681856] via 10.1.4.1, 00:04:53, Serial0/0 C 10.1.4.0/24 is directly connected, Serial0/0

Example 7-4 Seville Configuration and Routing Table After Route Summarization

Seville#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Seville(config)#interface serial 0/0

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

Seville(config-if)#^Z

Seville#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

Route Summarization and Variable-Length Subnet Masks 227

Example 7-4 Seville Configuration and Routing Table After Route Summarization (Continued)

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR

P - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks

D10.2.0.0/16 [90/2684416] via 10.1.6.1, 00:00:36, Serial0/0

D10.3.0.0/16 is a summary, 00:00:38, Null0

D10.1.1.0/24 [90/2195456] via 10.1.6.1, 00:00:36, Serial0/0 C 10.3.5.0/24 is directly connected, Loopback5

C 10.3.4.0/24 is directly connected, FastEthernet0/0 C 10.1.6.0/24 is directly connected, Serial0/0

C 10.3.7.0/24 is directly connected, Loopback7

D10.1.4.0/24 [90/2681856] via 10.1.6.1, 00:00:36, Serial0/0 C 10.3.6.0/24 is directly connected, Loopback6

Route summarization configuration differs with different routing protocols; EIGRP is used in this example. The summary routes for EIGRP are created by the ip summary-address interface subcommands on Yosemite and Seville in this case. Each command defines a new summarized route and tells EIGRP to only advertise the summary out this interface and not to advertise any routes contained in the larger summary. For instance, Yosemite defines a summary route to 10.2.0.0, mask 255.255.0.0, which defines a route to all hosts whose IP addresses begin with 10.2. In effect, this command causes Yosemite and Seville to advertise routes 10.2.0.0 255.255.0.0 and 10.3.0.0 255.255.0.0, respectively, and not to advertise their original four LAN subnets.

Notice that Albuquerque’s routing table now contains a route to 10.2.0.0 255.255.0.0 (the mask is listed in prefix notation as /16). None of the original four subnets that begin with

10.2 show up in Albuquerque’s routing table. The same thing occurs for route 10.3.0.0/16.

Also notice the highlighted section in Example 7-2 (Albuquerque) that says “variably subnetted”. This phrase means that more than one subnet mask is used in the network. Because of the summarization, Albuquerque knows some /24 routes and some /16 routes in network 10.0.0.0. So Albuquerque must use a routing protocol that supports VLSM for this to work. As it turns out, EIGRP supports VLSM.

The routing tables on Yosemite and Seville look a little different from Albuquerque. Focusing on Yosemite, notice that the four 10.2 routes show up because they are directly connected subnets. Yosemite does not see the four 10.3 routes. Instead, it sees a summary route, because Albuquerque now advertises the 10.3.0.0/16 summarized route only.