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228 Chapter 7: Advanced Routing Protocol Topics

The most interesting part of the routing tables is Yosemite’s route to, with the outgoing interface set to null0. Routes pointing out the null0 interface mean that packets matching this route are discarded. EIGRP added this route, with interface null0, as a result of the ip summary-address command. The logic works like this:

Yosemite needs this route because now it might receive packets destined for other 10.2 addresses besides the four existing 10.2 subnets. If a packet destined for one of the 4 existing 10.2.x subnets arrives, Yosemite has a correct, more specific route to match the packet. If a packet whose destination starts with 10.2 arrives, but it is not in one of those 4 subnets, the null route matches the packet, causing Yosemite to discard the packet—as it should.

The routing table on Seville is similar to Yosemite’s in terms of the table entries and why they are in the table.


VLSM occurs when more than one mask is used in a single Class A, B, or C network. Although route summarization causes more than one mask to be used, requiring support for VLSM, you can also simply design a network to use multiple subnet masks. By using VLSM, you can reduce the number of wasted IP addresses in each subnet, allow for more subnets, and avoid having to obtain another registered IP network number from the NIC.

Figure 7-2 depicts the same familiar network, but this time without summarization and with two different subnet masks used in network

Figure 7-2 VLSM in Network Masks and
















Mask: Except Where Shown

This figure shows a typical choice of using a /30 prefix (mask on point-to- point serial links, with some other mask ( in this example) on the LAN subnets. The only real requirements for VLSM are that the subnets do not overlap and that the routing protocol supports VLSM. Subnets overlap when the range of IP addresses in one subnet includes some addresses in the range of valid addresses in another subnet. When using a single mask in a single Class A, B, or C network, you can usually avoid overlapping subnets easily. However, with VLSM, you can more easily overlook cases in which you assign subnets that overlap.

Route Summarization and Variable-Length Subnet Masks 229

Example 7-5 lists the routing table on Albuquerque again, this time with the masks shown in Figure 7-2 and with summarization still configured on Yosemite and Seville.

Example 7-5 Albuquerque Routing Table with Two Separate Masks Configured on Interfaces, Plus Route Summarization

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 is variably subnetted, 5 subnets, 3 masks

D10.2.0.0/16 [90/2172416] via, 00:00:34, Serial0/0

D10.3.0.0/16 [90/2172416] via, 00:00:56, Serial0/1 C is directly connected, Ethernet0/0

C is directly connected, Serial0/1 C is directly connected, Serial0/0

Albuquerque uses VLSM with three masks now. Albuquerque knows the /24 and /30 prefixes (masks and, respectively) from the configuration of the connected interfaces. Albuquerque learns about the mask from the summarized routes advertised by Yosemite and Seville.

Regardless of how Albuquerque learns about these routes, to share these routes with other routers, it must use a routing protocol that supports VLSM. For a routing protocol to support VLSM, it must advertise not only the subnet number, but also the subnet mask, when advertising routes. Also, because VLSM support is required to support route summarization, the same routing protocols that support VLSM also support summarization. Table 7-2 lists the routing protocols and their support (or nonsupport) of VLSM.

Table 7-2 Interior IP Routing Protocol VLSM Support



Sends Mask/Prefix

Route Summarization

Routing Protocol

VLSM Support

in Routing Updates