60 Chapter 2: Deploying Interior Routing Protocols
With the preceding output, you can check that your OSPF network commands with the
wildcard masks are assigning interfaces to the expected areas (see the lines in boldface).
Finally, you want to ensure that routers are receiving and registering the OSPF routes. The
familiar show ip route command does this. The following is a partial listing:
RTB#sh ip ro
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, * - candidate default
U - per-user static route, o - ODR
T - traffic engineered route
Gateway of last resort is not set
10.0.0.0/30 is subnetted, 1 subnet
C 10.1.1.0 is directly connected, Serial1
18.104.22.168/16 is variably subnetted, 2 subnets, 2 masks
O 22.214.171.124/24 [110/74] via 126.96.36.199, 05:06:36, Serial0
C 188.8.131.52/30 is directly connected, Serial0
192.168.2.0/24 is subnetted, 1 subnet
O IA 192.168.2.0 [110/51] via 10.1.1.1, 05:06:26, Serial1
<lines deleted for brevity>
The preceding output for Router B contains two OSPF routes shown in boldface. Route
184.108.40.206/24 is coded with a lone letter O. This means the route is an intra-area route: The
route and Router B are part of the same area. Route 192.168.2.0/24, on the other hand, is coded
with O IA. This means the route is an inter-area route: The route is from a different area, not
attached to this router. The inter-area route originated from another area and arrived at Router
B through the backbone area, area 0.
NOTE You might notice from the key at the top of the show ip route output that there are other
kinds of OSPF routes. External routes are routes redistributed into OSPF (see Chapter 3). For
information on Not-So-Stubby-Area routes, see the IOS Conﬁguration Guide for IP Routing
This chapter covered basic theory and conﬁguration for the most common interior routing
protocols: RIP, IGRP, EIGRP, and OSPF. The next chapter, Chapter 3, builds on this baseline
study and focuses on how to manage these routing protocols.
The following are the key concepts of this chapter:
• Internetworking is the practice of connecting multiple individual networks so they
function as a single large internetwork or internet.
• Routing, which is the process of ﬁnding a path to a destination, makes
internetworking possible. It is the crucial network service that governs the ﬂow of
trafﬁc through your organization.
• A routing protocol is a language for routers. Routers use routing protocols to
exchange information about the topology and health of the internetwork.
• RIP, one of the oldest and simplest of routing protocols, is a classful, distance vector
protocol limited to a 15-hop metric. RIP generally does not scale well in large
• IGRP, a routing protocol invented by Cisco, is a classful, distance vector protocol with
a composite metric. IGRP supports autonomous systems and is designed to scale to
larger internets than RIP.
• EIGRP, another routing protocol invented by Cisco, is a classless, hybrid protocol
with a composite metric. EIGRP converges quickly, generates low trafﬁc overhead,
and scales well in large internets.
• Enabling RIP, IGRP, or EIGRP is fairly simple. To establish one of these services you
need two fundamental commands: router and network. For example, router eigrp
100 and network 172.16.0.0.
• OSPF is a classless, link-state protocol with a cost metric. OSPF converges quickly,
generates low trafﬁc overhead, and scales well in large internets. It requires more
system resources and more up-front planning than the other protocols.