configuration for area 1 in this example uses the
ip ospf 50 area 1 command. Alternatively, a separate
network router configuration command could have been
used.
Content 3.3 Implementing
and Verifying OSPF 3.3.4 Configuring a Router
ID For an OSPF routing process to start successfully, it
must be able to determine an OSPF router ID. The OSPF database
uses the OSPF router ID to uniquely describe each router in the
network. The OSPF routing process chooses a router ID for
itself when it starts up. The router ID is a unique IP address
that can be assigned in the following ways: - By
default, the highest IP address of any active physical
interface when OSPF starts is chosen as the router ID. The
interface does not have to be part of the OSPF process, but it
has to be up. There must be at least one up IP interface on the
router for OSPF to use as router ID. Otherwise, the following
error message occurs:
Router(config)#router ospf
1
2w1d: %OSPF-4-NORTRID: OSPF process 1 cannot start.
- Loopback interfaces never go down. If there is a
loopback interface, its IP address is always preferred as the
router ID instead of a physical interface address. This is true
even if the IP address of the physical interface is higher than
the IP address of the loopback interface. If there is more than
one loopback interface, the highest IP address on any loopback
interface becomes the router ID.
- Using the
router-id command is the preferred procedure to set the
router ID and is always used in preference to the other two
procedures.
Once the OSPF router ID is set, it does
not change, even if the interface that the router is using for
the router ID goes down. The OSPF router ID changes only if the
router is reloaded or if the OSPF routing process restarts. To
assign the OSPF router ID to a loopback address, first define a
loopback interface as follows: Router(config)#interface
loopback number OSPF is more reliable if a loopback
interface is configured, because the interface is always active
and cannot fail, as opposed to a real interface. For this
reason, you should use a loopback address on all key routers.
If the loopback address is advertised with the network
command, this address can be reached for testing purposes using
the ping command. A private IP address can be used to
save registered public IP addresses. Note
Loopback
addresses should be configured on a different subnet for each
router. If the OSPF process is already running, the router must
be reloaded or the OSPF process must be removed and
reconfigured for the new loopback address to take effect. Use
the OSPF router-id command to ensure that OSPF selects a
specific router ID. The ip-address parameter can be any
unique arbitrary 32-bit value in an IP address format. After
the router-id command is configured, use the clear ip
ospf process command. This command restarts the OSPF
routing process so that it will reselect the new IP address as
its router ID. Caution
The clear ip ospf
process command temporarily disrupts an operational
network.
Note
Router IDs have to be unique
throughout the autonomous system, no matter how they are
configured.
Content 3.3
Implementing and Verifying OSPF 3.3.5 Verifying
the OSPF Router ID Use the show ip ospf command to
verify the OSPF router ID. This command also displays OSPF
timer settings and other statistics, including the number of
times the SPF algorithm has been run. This command also has
optional parameters that allow you to specify the information
to be displayed. Figure shows a sample output for router B in
the previous example. Router B is an ABR.
Content
3.3 Implementing and Verifying
OSPF 3.3.6 Verifying the OSPF Operation To
verify that OSPF has been properly configured, use the
following show commands: - show ip
protocols—Displays IP routing protocol parameters about
timers, filters, metrics, networks, and other information for
the entire router.
- show ip route ospf—Displays
the OSPF routes known to the router. This command is one of
the most useful in determining connectivity between the local
router and the rest of the internetwork. Optional parameters
allow you to further specify the information to be displayed,
including the OSPF process ID.
- show ip ospf
interface—Verifies that interfaces are configured in the
intended areas. In addition, this command displays the timer
intervals (including the hello interval) and shows the neighbor
adjacencies.
- show ip ospf—Displays the OSPF
router ID, OSPF timers, the number of times the SPF algorithm
has been executed, and LSA information.
- show ip
ospf neighbor—Displays a list of neighbors, including their
OSPF router ID, their OSPF priority, their neighbor adjacency
state (for example, init, exstart, or full), and the dead
timer.
Content 3.3
Implementing and Verifying OSPF 3.3.7
Example of the show ip route ospf Command Use the show
ip route ospf command to verify the OSPF routes in the IP
routing table. In Figure , the O code represents OSPF routes,
and IA is “interarea.” The 10.2.1.0 subnet is recognized on
FastEthernet 0/0 via neighbor 10.64.0.2. The entry [110/782]
represents the administrative distance assigned to OSPF (110),
and the total cost of the route to subnet 10.2.1.0 (782).
Content 3.3 Implementing and Verifying
OSPF 3.3.8 Example of the show ip ospf
interface Command The show ip ospf interface
[type number] [brief] command displays
OSPF-related interface information. The command output in
Figure is from router A from the previous configuration
example and details the OSPF status of FastEthernet 0/0
interface. This command verifies that OSPF is running on this
particular interface and lists the OSPF area that it is in.
This command also displays other OSPF information, such as the
process ID, router ID, network type, DR and BDR, timers, and
neighbor adjacency.
Content 3.3
Implementing and Verifying OSPF 3.3.9 Example
of the show ip ospf neighbor Command One of the most
important OSPF troubleshooting commands is the show ip ospf
neighbor command. OSPF does not send or receive updates
without having full adjacencies established between neighbors.
The show ip ospf neighbor [type number]
[neighbor-id] [detail] command displays OSPF
neighbor information for each interface. Figure displays
information about the parameters of this command. In the output
in Figure , router B (from the previous configuration example)
has two neighbors. The first entry in the table represents the
adjacency formed on the Fast Ethernet interface. A FULL state
means that the LSDB has been exchanged successfully. The
DROTHER entry means that a router other than this neighboring
router is the DR. (Note that the OSPF priority on the router A
Fast Ethernet 0/0 interface has been set to 0, indicating that
it cannot be the DR or BDR on that interface.) The second line
in the table represents the neighbor of router B on the serial
interface. Point-to-point interfaces do not use DRs and BDRs
(indicated by a dash [–]). Figure shows a sample output of the
show ip ospf neighbor detail command, which provides
details of router B’s neighbors. Web Links OSPF
Neighbor Problems Explained
http://www.cisco.com/en/US/tech/tk365/
technologies_tech_note09186a0080094050.shtml
Content
3.3 Implementing and Verifying
OSPF 3.3.10 clear and debug Commands You
can use the following commands when troubleshooting OSPF. To
clear all routes from the IP routing table, use the following
command: Router#clear ip route * To clear a specific