Classless Routing Protocols and Classless Routing 233

Classless and Classful Routing Protocols

Some routing protocols must consider the Class A, B, or C network number that a subnet resides in when performing some of its tasks. Other routing protocols can ignore Class A, B, and C rules altogether. Routing protocols that must consider class rules are called classful routing protocols; those that do not need to consider class rules are called classless routing protocols.

You can easily remember which routing protocols fall into each category because of one fact:

Classful routing protocols do not transmit the mask information along with the subnet number, whereas classless routing protocols do transmit mask information.

You might recall that routing protocols that support VLSM do so because they send mask information along with the routing information. Table 7-3 lists the routing protocols and whether they transmit mask information, support VLSM, and are classless or classful.

Table 7-3 Interior IP Routing Protocol: Classless or Classful?

Classless routing protocols have an advantage over classful routing protocols because of their support of advanced features such as VLSM and summarization. Also, classless routing protocols overcome a few design issues only seen with classful routing protocols, as covered in the next short section.

Autosummarization

As covered earlier in this chapter, routers generally perform routing more quickly if the routing table size can be made shorter. Route summarization helps shorten the routing table while retaining all the needed routes in the network.

Because classful routing protocols do not advertise subnet mask information, the routing updates simply have numbers in them representing the subnet numbers, but no accompanying mask. A router receiving a routing update with a classful routing protocol looks at the subnet number in the update and “guesses” the correct mask. For instance, with Cisco routers, if R1 and R2 have connected networks of the same single Class A, B, or C

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network, and if Router B receives an update from R1, R2 assumes that the routes described in R1’s update use the same mask that R2 uses. In other words, the classful routing protocols expect a static-length subnet mask (SLSM) throughout the network, because they can then reasonably assume that the mask configured for their own interfaces is the same mask used throughout the network.

When a router has interfaces in more than one Class A, B, or C network, it advertises a single route for an entire Class A, B, or C network into the other network. This feature is called autosummarization. It can be characterized as follows:

When advertised on an interface whose IP address is not in network X, routes related to subnets in network X are summarized and advertised as one route. That route is for the entire Class A, B, or C network X.

In other words, if R3 has interfaces in networks 10.0.0.0 and 11.0.0.0, when R3 advertises routing updates out interfaces with IP addresses that start with 11, the updates advertise a single route for network 10.0.0.0. Similarly, R3 advertises a single route to 11.0.0.0 out its interfaces whose IP addresses start with 10.

RIP and IGRP perform autosummarization by default. It cannot be disabled—it is simply a feature of classful routing protocols. (For RIP-2 and EIGRP, autosummarization can be enabled or disabled.)

As usual, an example makes the concept much clearer. Consider Figure 7-3, which shows two networks in use: 10.0.0.0 and 172.16.0.0. Seville has four (connected) routes to subnets of network 10.0.0.0. Example 7-6 shows the output of the show ip route command on Albuquerque, as well as RIP-1 debug ip rip output.

Figure 7-3 Autosummarization

Classless Routing Protocols and Classless Routing 235

Example 7-6 Seville Configuration

Albuquerque#show ip route

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

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

172.16.0.0/24 is subnetted, 2 subnets

C172.16.1.0 is directly connected, Ethernet0/0

C172.16.3.0 is directly connected, Serial0/1

R10.0.0.0/8 [120/1] via 172.16.3.3, 00:00:28, Serial0/1

Albuquerque#debug ip rip

RIP protocol debugging is on

00:05:36: RIP: received v1 update from 172.16.3.3 on Serial0/1 00:05:36: 10.0.0.0 in 1 hops

As shown in Example 7-6, Albuquerque’s received update on Serial0/1 from Seville advertises only the entire Class A network 10.0.0.0 because autosummarization is enabled on Seville (by default). The Albuquerque IP routing table lists just one route to network 10.0.0.0.

This example also points out another feature of how classful routing protocols make assumptions. Albuquerque does not have any interfaces in network 10.0.0.0. So, when Albuquerque receives the routing update, it assumes that the mask used with 10.0.0.0 is 255.0.0.0—the default mask for a Class A network. In other words, classful routing protocols expect autosummarization to occur.

Autosummarization does not cause any problems—as long as network 10.0.0.0 is contiguous. Consider Figure 7-4, in which Yosemite also has subnets of network 10.0.0.0 but has no connectivity to Seville other than through Albuquerque.

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Figure 7-4 Autosummarization Pitfalls

172.16.1.0

Mask: 255.255.255.0

IP subnet design traditionally has not allowed discontiguous networks. A contiguous network is a single Class A, B, or C network for which all routes to subnets of that network pass through only other subnets of that same single network. Discontiguous networks refers to the concept that, in a single Class A, B, or C network, there is at least one case in which the only routes to one subnet pass through subnets of a different network. An easy analogy for residents of the U.S. is the term contiguous 48, referring to the 48 states besides Alaska and Hawaii. To drive to Alaska from the contiguous 48, for example, you must drive through another country (Canada, for the geographically impaired!), so Alaska is not contiguous with the 48 states. In other words, it is discontiguous.

Figure 7-4 shows the discontiguous network 10.0.0.0. Simply put, classful routing protocols do not support a design with discontiguous networks, but classless routing protocols do support discontiguous networks. Example 7-7 shows the network shown in Figure 7-4, with classful RIP causing confused routing at Albuquerque.

Example 7-7 Albuquerque Routing Table: Classful Routing Protocol Not Allowing Discontiguous Network

Albuquerque#show ip route

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

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

172.16.0.0/24 is subnetted, 3 subnets

C172.16.1.0 is directly connected, Ethernet0/0

C172.16.2.0 is directly connected, Serial0/0

C172.16.3.0 is directly connected, Serial0/1

R10.0.0.0/8 [120/1] via 172.16.3.3, 00:00:13, Serial0/1

[120/1] via 172.16.2.2, 00:00:04, Serial0/0

Classless Routing Protocols and Classless Routing 237

As shown in Example 7-7, Albuquerque now has two routes to network 10.0.0.0. Instead of sending packets destined for Yosemite’s subnets out serial 0/0, Albuquerque sends some packets out S0/1 to Seville! Albuquerque simply balances the packets across the two routes, because as far as Albuquerque can tell, the two routes are simply equal-cost routes to the same destination—the entire network 10.0.0.0. So, applications would cease to function correctly in this network.

Migrating to use a classless routing protocol with autosummarization disabled takes care of this problem. Example 7-8 shows the same network, this time with EIGRP configured and no autosummarization.

Example 7-8 Albuquerque Routing Table: Classless Routing Protocol Allowing Discontiguous Network

Albuquerque#show ip route

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

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

172.16.0.0/24 is subnetted, 3 subnets

C172.16.1.0 is directly connected, Ethernet0/0

C172.16.2.0 is directly connected, Serial0/0

C172.16.3.0 is directly connected, Serial0/1 10.0.0.0/24 is subnetted, 8 subnets

D10.2.1.0/24 [90/2172416] via 172.16.2.2, 00:00:01, Serial0/0

D10.2.2.0/24 [90/2297856] via 172.16.2.2, 00:00:01, Serial0/0

D10.2.3.0/24 [90/2297856] via 172.16.2.2, 00:00:01, Serial0/0

D10.2.4.0/24 [90/2297856] via 172.16.2.2, 00:00:01, Serial0/0

D10.3.5.0/24 [90/2297856] via 172.16.3.3, 00:00:29, Serial0/1

D10.3.4.0/24 [90/2172416] via 172.16.3.3, 00:00:29, Serial0/1

D10.3.7.0/24 [90/2297856] via 172.16.3.3, 00:00:29, Serial0/1

D10.3.6.0/24 [90/2297856] via 172.16.3.3, 00:00:29, Serial0/1

Notice that Albuquerque knows the four LAN subnets off Yosemite, as well as the four LAN subnets off Seville. Because EIGRP is classless, it can transmit the mask with the routes. Interestingly, EIGRP performs autosummarization by default, but in this configuration, EIGRP autosummarization is disabled. Had autosummarization still been enabled, this network design would have the same problem with the discontiguous network 10.0.0.0, as shown in Example 7-7.