© 2002, Cisco Systems, Inc. All rights reserved.
© 2002, Cisco Systems, Inc. All rights reserved.
2
Enabling OSPF
Objectives
Upon completing this lesson, you will be able to:
Describe the features and operation of OSPF
Use Cisco IOS commands to configure dynamic routing for a single area OSPF network, given a functioning router
Use show and debug commands to identify anomalies in dynamic routing operation using OSPF, given an operational router
Introducing OSPF
Open standard
Shortest path first (SPF) algorithm
Link-state routing protocol (vs. distance vector)
OSPF as a Link-State Protocol
OSPF propagates link-state advertisements rather than routing table updates.
LSAs are flooded to all OSPF routers in the area.
The OSPF link-state database is pieced together from the LSAs generated by the OSPF routers.
OSPF uses the SPF algorithm to calculate the shortest path to a destination.
Link = router interface
State = description of an interface and its relationship to neighboring routers
OSPF Hierarchical Routing
Consists of areas and autonomous systems
Minimizes routing update traffic
Shortest Path First Algorithm
Places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost
Cost = 108/bandwidth (bps)
Configuring Single Area OSPF
Router(config-router)#network address mask area area-id
Assigns networks to a specific OSPF area
Router(config)#router ospf process-id
Defines OSPF as the IP routing protocol
OSPF Configuration Example
Configuring Loopback Interfaces
Router ID:
Number by which the router is known to OSPF
Default: The highest IP address on an active interface at the moment of OSPF process startup
Can be overridden by a loopback interface: Highest IP address of any active loopback interface
Router#show ip ospf interface
Verifying the OSPF Configuration
Displays area-ID and adjacency information
Router#show ip protocols
Verifies that OSPF is configured
Router#show ip route
Displays all the routes learned by the router
Router#show ip ospf neighbor
Displays OSPF-neighbor information on a per-interface basis
OSPF debug commands
Router#debug ip ospf events

OSPF:hello with invalid timers on interface Ethernet0
hello interval received 10 configured 10
net mask received 255.255.255.0 configured 255.255.255.0
dead interval received 40 configured 30
Router# debug ip ospf packet

OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.117
aid:0.0.0.0 chk:6AB2 aut:0 auk:

Router#debug ip ospf packet

OSPF: rcv. v:2 t:1 l:48 rid:200.0.0.116
aid:0.0.0.0 chk:0 aut:2 keyid:1 seq:0x0
Summary
OSPF is an interior gateway protocol similar to IGRP, but based on link states rather than distance vectors.
OSPF advertises information about each of its links rather than sending routing table updates like a distance vector protocol.
The SPF algorithm places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost required to reach that destination.
Summary (Cont.)
Use the router ospf command to start an OSPF routing process and the network command to associate addresses to an OSPF area.
You can use any one of a number of show commands to display information about an OSPF configuration.
To display information on OSPF-related events, such as adjacencies, flooding information, designated router selection, and SPF calculation, use the debug ip ospf events privileged EXEC command.