An AR Router Fails to Establish an OSPF Adjacency with a Non-Huawei Device

Publication Date:  2015-10-14 Views:  373 Downloads:  0
Issue Description
As shown in Figure 1-1, an AR3260 connects to a non-Huawei device. During the OSPF adjacency establishment, the state machine remains the Exstart state and cannot reach the Full state. As a result, routes cannot be learned correctly. During the query of information about packets obtained on the port that connects the AR3260 to the non-Huawei device, only Hello and database description (DD) packets are found.

Figure 1-1 An AR Router Fails to Establish an OSPF Adjacency with a Non-Huawei Device

Handling Process
Step 1 Check the AR3260 configuration.

Run the display current-configuration command on the AR3260 to check the current parameter settings.
#
interface GigabitEthernet0/0/1                                    
ip address 10.2.1.2 255.255.255.0                                
ospf mtu-enable   
#

The configuration file shows that the MTU detection function has been enabled on the interface that connects the AR3260 to the peer device. After this function is enabled, the interface will send DD packets that carry the MTU and check whether the MTU carried in the DD packets received from the peer interface exceeds the local MTU. If so, the OSPF adjacency remains the Exstart state and cannot reach the Full state.

Step 2 Run the undo ospf mtu-enable command on the AR3260 to set the MTU carried in the DD packets to be sent by the interface to 0. That is, disable the MTU detection function. Subsequently, the interface does not check the MTU when establishing an OSPF adjacency with the peer interface. After the configuration is complete, the two devices can establish an OSPF adjacency, and the problem is solved.
Root Cause
The MTU detection function has been enabled on the AR3260. When establishing an OSPF adjacency, the AR3260 checks whether the local interface uses the consistent MTU with the peer interface. If not, the OSPF adjacency remains the Exstart state.
Solution
During the OSPF adjacency establishment, disable the MTU detection function. By default, an AR device can establish an OSPF adjacency with the peer device when the MTU detection function is disabled.

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