To be able to take advantage of VLSM and discontiguous
network addressing we must use a routing protocol that will carry a
subnet mask. Such a routing protocol is called classless.
Examples of classless routing protocols include Open Shortest
Path First (OSPF), RIPv2, and Enhanced Interior Gateway Routing
Protocol (EIGRP).
Let us take another look at the ARI scenario from Chapter 7 (Figure 8.1).
Given a single IP address and a classful routing protocol,
there would be no way of configuring all hosts of ARI.
As a result, a classless routing protocol must be
used.
Figure 8.1 ARI Topology
Figure 8.2 shows a portion of an OSPF debug. This output
illustrates how router SANFRAN received updates relating to the
serial connections and mask to TAMPA and DALLAS.
SANFRAN# debug ip ospf events
OSPF: Add Network Route to 192.16.15.8
Mask /30. Metric: 65, Next Hop: 192.16.15.5
OSPF: Add Network Route to 192.16.15.12
Mask /30. Metric: 65, Next Hop: 192.16.15.5
Figure 8.2 OSPF Debug
Figures 8.3 and 8.4 show the OSPF and the EIGRP routing
tables respectively, of JAX.
Router JAX “learned” all of the subnets of the 198.10.11.0
network.
JAX#sh 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
O 10.0.1.0 [110/74] via 172.16.1.1,
00:02:41, Serial1
O 10.0.4.0 [110/74] via 172.16.2.1,
00:02:41, Serial0
Figure 8.6 OSPF Routing Table
Even though network 172.16.0.0 is in between network 10.0.0.0
Router_A can still distinguish which interface to use to get to
either subnet of network 10.0.0.0.
A common question is “Which is the best routing protocol,
OSPF or EIGRP?”
Keep in mind that OSPF is an open routing protocol, meaning
that all vendors support OSPF. EIGRP on the other hand is a
proprietary protocol belonging to Cisco. You must have all
networking devices running the Cisco IOS to support EIGRP.
We will configure OSPF and EIGRP in later chapters.