OSPF Neighbor States


This section discusses the methods of troubleshooting problems that arise while establishing neighbor relationships in the OSPF routing process. Table 9.3 lists some common situations encountered while troubleshooting OSPF neighbor adjacency problems. The table also shows the commands used to verify the problems.

Table 9.3 : Causes of Neighbor Adjacency Problems and Diagnostic Commands

Cause of Neighbor Adjacency Problems

Diagnostic Command

OSPF is not configured on any of the routers in the network.

show ip ospf

OSPF is not configured on the needed interface.

show ip ospf interface

Neighbor routers are incorrectly configured.

show ip ospf

show ip ospf interface

OSPF Hello packets are not

processed.

show memory summary

show memory processor

Consider the example depicted in Figure 9.3. This figure shows part of an OSPF network with a single area. Routers A1 and A2 belong to the same LAN, 10.0.1.0/24.

click to expand
Figure 9.3: An OSPF network showing routers A1 and A2.

In Figure 9.3, A2 is not able to establish a neighbor relationship with A1, as ascertained with the show ip ospf neighbor command.

Run the show ip ospf neighbor command many times in A2 to ascertain that there is no change in the state of A1. This command can be used to find out the

possible problem areas in the neighbor state. The different neighbor states in OSPF are:

  • INIT state

  • EXCHANGE/EXSTART state

  • 2-way state

  • Nothing at all state

  • Loading state

Let us discuss the problems encountered in each of these neighbor states.

INIT State

The INIT state implies that A1 has been able to detect Hello packets from A2, but A2 has not been able to detect Hello packets from A1. If establishing the neighbor relationship is stuck at the INIT state, the output of the show ip ospf neighbor command is as shown in Listing 9.13.

Listing 9.13 Output of show ip ospf neighbor Command in INIT State

start example
A2#show ip ospf neighbor Neighbor  ID  Pri  State  Dead Time  Address  Interface 0 10.10.1.2   1  INIT  00:00:31  10.0.1.2  FastEthernet0/1
end example

To troubleshoot OSPF neighbors in INIT state:

  1. Check if OSPF authentication is turned on for all OSPF routers in the LAN by using the show ip ospf interface command.

  2. Check if the authentication type and authentication key match across the LAN.

  3. Check and modify any access lists hampering the flow of hello packets by using the show ip interface command.

  4. Check the switch configuration and physical cabling of the LAN.

EXCHANGE or EXSTART State

In the EXSTART state, bidirectional communication with the neighbor is already established. The routers try to establish the initial sequence number that would be used in the exchange of information. In the EXCHANGE state, routers exchange the complete link state database by sending database description packets.

If establishing the neighbor relationship is stuck in EXCHANGE or EXSTART state, the output of the show ip ospf neighbor command is as shown in Listing 9.14.

Listing 9.14 Output of show ip ospf neighbor Command in EXCHANGE State

start example
A2#show ip ospf neighbor Neighbor ID     Pri  State  Dead Time  Address  Interface 0 10.10.1.2   1   EXCHANGE     00:00:31  0.0.1.2  FastEthernet0/1
end example

To troubleshoot OSPF neighbors:

  1. Check the MTU size using the show ip interface command. This parameter should be the same for potential neighbors.

  2. Check the pinging of neighboring interfaces with a large-sized packet, if MTU is OK.

  3. Troubleshoot the physical and software levels in Layer 2.

2-Way State

In the 2-way state, a bidirectional communication is established with a neighbor. DR and BDR election takes place at the end of the 2-way state. Full adjacencies are to be built only by the DR and BDR with the other routers, in the case of a broadcast network. If establishing neighbor relationships is stuck at 2-way state, the output of the show ip ospf neighbor command is as shown in Listing 9.15.

Listing 9.15 Output of show ip ospf neighbor Command in 2-way State

start example
A2#show ip ospf neighbor Neighbor ID     Pri     State     Dead Time     Address     Interface 0 10.10.1.2   1   2WAY/DROTHER     00:00:31     10.0.1.2 FastEthernet0/1
end example

Note

Remaining stuck in 2-way state is normal behavior for routers forming neighbor relationship with non-DR and non-BDR routers.

Nothing at All State

In the nothing at all state, the show ip ospf neighbor command gives no output. To troubleshoot OSPF routers in the nothing at all state:

  1. Check the common interface between the router and potential neighbors by using the show ip interface FastEthernet0/1 command. The output of the show ip interface FastEthernet0/1 command is shown in Listing 9.16.

Listing 9.16 Output of show ip interface FastEthernet0/1 Command at A2

start example
A2#show ip interface FastEthernet0/1 FastEthernet0/1 is up, line protocol is down Internet address is 10.0.1.1/30 Broadcast address is 255.255.255.255 Address determined by non-volatile memory MTU is 1500 bytes Helper address is not set Directed broadcast forwarding is disabled Multicast reserved groups joined: 224.0.0.10 Outgoing access list is not set Inbound access list is not set Proxy ARP is enabled Security level is default Split horizon is enabled ICMP redirects are never sent ICMP unreachables are always sent ICMP mask replies are never sent IP fast switching is enabled IP fast switching on the same interface is enabled IP Flow switching is disabled IP Feature Fast switching turbo vector IP multicast fast switching is enabled IP multicast distributed fast switching is disabled IP route-cache flags are Fast Router Discovery is disabled IP output packet accounting is disabled IP access violation accounting is disabled TCP/IP header compression is disabled RTP/IP header compression is disabled Probe proxy name replies are disabled Policy routing is disabled Network address translation is enabled, interface in domain inside WCCP Redirect outbound is disabled WCCP Redirect exclude is disabled BGP Policy Mapping is disabled
end example

In Listing 9.16, the code FastEthernet0/1 is up, line protocol is down shows there is a problem with the physical connectivity of the switch configuration. This problem should be rectified to establish neighbor relationships.

  1. Check if the output of the show ip interface FastEthernet0/1 command is as shown.

    FastEthernet0/1 is administratively down, line protocol is down implies that the FastEthernet interface has to be manually unshut using the commands:

    A2(config)#interface FastEthernet0/1 A2(config-if)#no shut
  2. Check if the output of the show ip interface FastEthernet0/1 command is showing the status as:

    FastEthernet0/1 is up, line protocol is up

    In this case, ping testing is to be performed using the A2#ping 10.10.1.2 command.

  3. Check for the presence of any access lists applied to the FastEthernet interfaces in both routers, if the ping command is not successful.

  4. Check the quality of the physical link if the ping command is still unsuccessful.

  5. Check whether OSPF is configured for the interfaces at both routers by using the A2#show ip ospf interface fastEthernet0/1 command, if the ping command is successful.

  6. Configure OSPF using the commands:

    A2(config)#router ospf 1 A2(config-router)#network 10.0.1.0 0.0.0.255

    If OSPF is not configured at the router interfaces.

  7. Check if the routers are configured as passive interfaces by using the A2#show ip ospf interface fastEthernet0/1 command.

  8. Remove passive interface configuration, because OSPF does not exchange Hello packets to form neighbor relationships over a passive interface.

  9. Check if the network type is broadcast for all the members connected in a LAN. Check if the subnet mask is same for all the routers in the broadcast network segment by using the A2#show ip ospf interface fastEthernet0/1 command.

  10. Check if neighboring interfaces are configured in the same area by using the A2#show ip ospf interface fastEthernet0/1 command.

  11. Check if the values of dead and hello timers match between neighboring interfaces by using the A2#show ip ospf interface fastEthernet0/1 command.

  12. Use the A2#debug ip ospf adjacency command to find out more information.

Load State

In a loading state, the information exchange between the OSPF routers that have formed adjacency is finalized. If a router receives an outdated or incomplete LSA during adjacency formation, it is out in the request list. These are sent out as link state requests. The requested information is sent by the requested neighbor in the form of link state updates.

A2#show ip ospf neighbor Neighbor ID     Pri     State     Dead Time     Address     Interface 0 10.10.1.2   1   LOADING     00:00:31     10.0.1.2 FastEthernet0/1

If the neighbor router is stuck in loading state, follow these steps:

  1. Find the link state requests that are pending reply by using the A2#show ip ospf request-list command.

  2. Check for the %OSPF-4-BADLSA message in the console. When the neighbor router is stuck in the loading state, it may exchange corrupt LSAs.

  3. Check the compatibility of OSPF implementation in neighbor routers to detect the cause of corrupt LSAs.




Cisco IP Routing Protocols(c) Trouble Shooting Techniques
Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)
ISBN: 1584503416
EAN: 2147483647
Year: 2006
Pages: 130

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