Verifying OSPF

The show ip ospf neighbor command can be used to verify and troubleshoot OSPF neighbor relationships. For each neighbor, this command displays the following output:
Neighbor ID - The router ID of the neighboring router.
Pri - The OSPF priority of the interface. This is discussed in a later section.
State - The OSPF state of the interface. FULL state means that the router and its neighbor have identical OSPF link-state databases. OSPF states are discussed in CCNP.
Dead Time - The amount of time remaining that the router will wait to receive an OSPF Hello packet from the neighbor before declaring the neighbor down. This value is reset when the interface receives a Hello packet.
Address - The IP address of the neighbor's interface to which this router is directly connected.
Interface - The interface on which this router has formed adjacency with the neighbor.
When troubleshooting OSPF networks, the show ip ospf neighbor command can be used to verify that the router has formed an adjacency with its neighboring routers. If the router ID of the neighboring router is not displayed, or if it does not show as a state of FULL, the two routers have not formed an OSPF adjacency. If two routers do not establish adjacency, link-state information will not be exchanged. Incomplete link-state databases can cause inaccurate SPF trees and routing tables. Routes to destination networks may either not exist or may not be the most optimum path.
Note: On multiaccess networks such as Ethernet, two routers that are adjacent may have their states displayed as 2WAY. This will be discussed in a later section.
Two routers may not form an OSPF adjacency if:
The subnet masks do not match, causing the routers to be on separate networks.
OSPF Hello or Dead Timers do not match.
OSPF Network Types do not match.
There is a missing or incorrect OSPF network command.
Other powerful OSPF troubleshooting commands include:
show ip protocols
show ip ospf
show ip ospf interface
As shown in the figure, the show ip protocols command is a quick way to verify vital OSPF configuration information, including the OSPF process ID, the router ID, networks the router is advertising, the neighbors the router is receiving updates from, and the default administrative distance, which is 110 for OSPF.
The show ip ospf command can also be used to examine the OSPF process ID and router ID. Additionally, this command displays the OSPF area information as well as the last time the SPF algorithm was calculated. As you can see from the sample output, OSPF is a very stable routing protocol. The only OSPF-related event that R1 has participated in during the past 11 and a half hours is to send small Hello packets to its neighbors.
Note: Additional information displayed by the show ip ospf command is discussed in CCNP courses.
The command output includes important SPF algorithm information which includes the SPF schedule delay:
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Any time a router receives new information about the topology (addition, deletion, or modification of a link), the router must rerun the SPF algorithm, create a new SPF tree, and update the routing table. The SPF algorithm is CPU-intensive and the time it takes for calculation depends on the size of the area. The size of an area is measured by the number of routers and the size of the link-state database.
A network that cycles between an up state and a down state is referred to as a flapping link. A flapping link can cause OSPF routers in an area to constantly recalculate the SPF algorithm, preventing proper convergence. To minimize this problem, the router waits 5 seconds (5000 msecs) after receiving an LSU before running the SPF algorithm. This is known as the SPF schedule delay. In order to prevent a router from constantly running the SPF algorithm, there is an additional Hold Time of 10 seconds (10000 msecs). The router waits 10 seconds after running the SPF algorithm before rerunning the algorithm again.
The quickest way to verify Hello and Dead intervals is to use the show ip ospf interface command. As shown in the figure, adding the interface name and number to the command displays output for a specific interface. These intervals are included in the OSPF Hello packets sent between neighbors. OSPF may have different Hello and Dead intervals on various interfaces, but for OSPF routers to become neighbors, their OSPF Hello and Dead intervals must be identical. For example, in the figure, R1 is using a Hello interval of 10 and a Dead interval of 40 on the Serial 0/0/0 interface. R2 must also use the same intervals on its Serial 0/0/0 interface or the two routers will not form an adjacency.


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