What command would be useful to determine why OSPF is not forming adjacencies

The first component to troubleshoot and verify is the OSPF neighbor adjacency. Figure 4-9 shows the verification/troubleshooting components for neighbor adjacencies.

Inhaltsverzeichnis Show

How does OSPF form adjacencies?
What command can you use to verify the neighbor adjacencies of an OSPF router?
What is the command you can use to see OSPF adjacency ?*?
What must match for OSPF neighbors to form adjacencies?

Figure 4-9 Troubleshooting OSPF Neighbor Adjacencies

A healthy OSPF neighbor state is "Full." If the OSPF neighbor state remains in any other state, it may indicate a problem. Example 4-12 demonstrates sample output from the show ip ospf neighbor command to gather this information.

Example 4-12 Verifying OSPF Neighbor State

RouterX#show ip

ospf

neighbor

Neighbor ID

Pri

State

Dead

Time

Address

Interface

172.16.31.100

Full/ -

00:0

:31

10.140.1.1

Serial0/0/0

192.168.1.81

Full/ -

00:0

:31

10.23.23.2

Serial0/0/1

To determine whether a possible Layer 1 or Layer 2 problem exists with a connection, display the status of an interface using the show ip ospf neighbor command. "Administratively Down" indicates that the interface is not enabled. If the status of the interface is not up/up, there will be no OSPF neighbor adjacencies. In Example 4-13, serial 0/0/1 is up/up.

Example 4-13 Verifying Interface Status

RouterX#show ip ospf interface

Serial0/0/1 is up, line protocol is up Internet Address 10.23.23.1/24, Area 0

Process ID 100, Router ID 192.168.1.65, Network Type POINT_TO_POINT, Cost: 1562

For OSPF to create an adjacency with a directly connected neighbor router, both routers must agree on the maximum transmission unit (MTU) size. To check the MTU size of an interface, use the show interface command. In Example 4-14, the MTU size is 1500 bytes.

Example 4-14 Verifying Interface MTU Size

RouterX#show ip interface fa0/0

FastEthernet0/0 is up, line protocol is up Internet address is 10.2.2.3/24 Broadcast address is 255.255.255.255 Address determined by setup command MTU is 1500 bytes Helper address is not set Directed broadcast forwarding is disabled Outgoing access list is not set Inbound access list is not set

The network command that you configure under the OSPF routing process indicates which router interfaces participate in OSPF and determines in which area the interface belongs. If an interface appears under the show ip ospf interface command, that interface is running OSPF. In Example 4-15, interfaces serial 0/0/1 and serial 0/0/0 are running OSPF.

Example 4-15 Verifying Whether an Interface Is Running OSPF

RouterX#show ip ospf interface

Serial0/0/1 is up, line protocol is up

Internet Address 10.23.23.1/24, Area 0

Process ID 100, Router ID 192.168.1.65, Network Type POINT_TO_POINT, Cost:

1562

Transmit Delay is 1 sec, State POINT_TO_POINT,

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

oob-resync timeout 40

Hello due in 00:00:04

Neighbor Count is 1, Adjacent neighbor count is 1

Adjacent with neighbor 192.168.1.81

Suppress hello for 0 neighbor(s)

Simple password authentication enabled

Serial0/0/0 is up, line protocol is up

Internet Address 10.140.1.2/24, Area 0

Process ID 100, Router ID 192.168.1.65, Network Type POINT_TO_POINT, Cost:

1562

Transmit Delay is 1 sec, State POINT_TO_POINT,

OSPF routers exchange hello packets to create neighbor adjacencies. Four items in an OSPF hello packet must match before an OSPF adjacency can occur:

■ Hello and dead intervals

■ Authentication password

To determine whether any of these hello packet options do not match, use the debug ip ospf adj command. The output in Example 4-16 illustrates a successful adjacency on the serial 0/0/1 interface.

Example 4-16 Verifying OSPF Adjacencies

*Feb 17 18:41:51.242: OSPF: Interface Serial0/0/1 going Up

*Feb 17 18:41:51.742: OSPF: Build router LSA for area 0, router ID 10.1.1.1, seq 0x80000013 *Feb 17 18:41:52.242: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/1, changed state to up

*Feb 17 18:42:01.250: OSPF: 2 Way Communication to 10.2.2.2 on Serial0/0/1, state 2WAY *Feb 17 18:42:01.250: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0x9B6 opt 0x52 flag 0x7 len 32

*Feb 17 18:42:01.262: OSPF: Rcv DBD from 10.2.2.2 on Serial0/0/1 seq 0x23ED opt0x52 flag 0x7

len 32 mtu 1500 state EXSTART *Feb 17 18:42:01.262: OSPF: NBR Negotiation Done. We are the SLAVE

*Feb 17 18:42:01.262: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0x23ED opt 0x52 flag 0x2 len 72

*Feb 17 18:42:01.294: OSPF: Rcv DBD from 10.2.2.2 on Serial0/0/1 seq 0x23EE opt0x52 flag 0x3

len 72 mtu 1500 state EXCHANGE *Feb 17 18:42:01.294: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0x23EE opt 0x52 flag 0x0 len 32

*Feb 17 18:42:01.294: OSPF: Database request to 10.2.2.2

*Feb 17 18:42:01.294: OSPF: sent LS REQ packet to 192.168.1.102, length 12

*Feb 17 18:42:01.314: OSPF: Rcv DBD from 10.2.2.2 on Serial0/0/1 seq 0x23EF opt0x52 flag 0x1

len 32 mtu 1500 state EXCHANGE *Feb 17 18:42:01.314: OSPF: Exchange Done with 10.2.2.2 on Serial0/0/1

*Feb 17 18:42:01.314: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0x23EF opt 0x52 flag 0x0 len 32

*Feb 17 18:42:01.326: OSPF: *Feb 17 18:42:01.330: %OSPF

from LOADING to FULL, Loading Done *Feb 17 18:42:01.830: OSPF: Build router LSA for area 0, router ID 10.1.1.1, seq 0x80000014

Synchronized with 10.2.2.2 on Serial0/0/1, state FULL -5-ADJCHG: Process 10, Nbr 10.2.2.2 on Serial0/0/1

How does OSPF form adjacencies?

To establish OSPF full adjacency, two neighbor routers must be in the same area. Any individual interface can only be attached to a single area. If the address ranges specified for different areas overlap, IOS will adopt the first area in the network command list and ignore subsequent overlapping portions.

What command can you use to verify the neighbor adjacencies of an OSPF router?

To verify the OSPF adjacencies, use the show ip ospf neighbor command as shown in Figure 1. Unlike serial links that only display a state of FULL/- , the state of neighbors in multiaccess networks can be: FULL/DROTHER - This is a DR or BDR router that is fully adjacent with a non-DR or BDR router.

What is the command you can use to see OSPF adjacency ?*?

Displays OSPF neighbor information.

What must match for OSPF neighbors to form adjacencies?

OSPF Neighborship Requirements From the perspective of OSPF, there are a couple of things that must match for a OSPF neighborship to establish; these include: The devices must be in the same area. The devices must have the same authentication configuration. The devices must be on the same subnet.