What are common reasons why an OSPF neighbor relationship cannot change to full state

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Common causes of this problems are:

The OSPF network is a non-broadcast multiple access (NBMA) network, and no neighbor is configured.
The interval at which the OSPF neighbor sends Hello packets is longer than the aging time of the neighbor relationship.
In a stub area or not-so-stubby area (NSSA) area, some devices are not configured as stub routers or NSSA routers.
The configuration of OSPF verification is incorrect.
The OSPF router IDs on two ends of the OSPF link are different.
The network types on two ends of the OSPF link are different.
The MTU values on two ends of the OSPF link are different.
OSPF is not enabled on the local or remote device.
The area numbers on two ends of the OSPF link are different.

Other related questions:
Common reasons why an OSPF neighbor relationship between S series switches cannot change to the Full state
Question: What are common reasons why an OSPF neighbor relationship cannot change to the Full state? Answer: Common causes of this problem include the following: 1. The OSPF network is a non-broadcast multiple access (NBMA) network, and no neighbor is configured. 2. The interval at which the OSPF neighbor sends Hello packets is longer than the aging time of the neighbor relationship. 3. In a stub area or not-so-stubby area (NSSA) area, some switches are not configured as stub or NSSA. 4. The configuration of OSPF verification is incorrect. 5. The OSPF router IDs on two ends of the OSPF link are the same. 6. The network types on two ends of the OSPF link are different. 7. The MTU values on two ends of the OSPF link are different. 8. OSPF is not enabled on the local or remote device. 9. The area numbers on two ends of the OSPF link are different.

Problem and solution when the OSPF status is abnormal
To solve the problem that the OSPF status between the firewall and the peer device cannot reach the Full state, perform the following steps: 1. Check the OSPF status. Check whether the OSPF neighboring relationship can be established between the firewall and the peer device. 2. If no, check the security policy configuration. Check whether the security policy control function for unicast packets is enabled. That is, check whether the firewall packet-filter basic-protocol enable command is configured. If yes, run the undo firewall packet-filter basic-protocol enable command to disable the function. To establish an OSPF neighboring relationship, devices need to exchange DD packets. DD packets are OSPF unicast packets. By default, the forwarding of OSPF unicast packets is not controlled by security policies. However, if you run the firewall packet-filter basic-protocol enable command to enable the security policy control function for OSPF unicast packets, you need also to configure the corresponding security policy to allow the packets to be forwarded. For details, see OSPF can not step into full state caused by security policy deny.

Why does an OSPF neighbor relationship between two S series switches remain in the 2-Way state
Question: Why are Full/DR, FULL/BDR, and 2-WAY/DR OTHER displayed in the display ospf peer command output? A: To reduce broadcast network (such as the Ethernet, FDDI, and token ring network) media to be flooded, only the DR and BDR can establish Full neighbor relationships with DR Others. The neighbor relationship between two DR Others remains in the 2-Way state.

How to configure OSPF on S series switches
For OSPF configuration on S series switches, see Common OSPF Operations on S Series Switches. For typical OSPF configuration examples on S series switches, see "Typical Routing Configuration - Typical OSPF Configuration" in S1720&S2700&S3700&S5700&S6700&S7700&S9700 Typical Configuration Examples.

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