The most common causes are as follows:




1.The OSPF network type is non-broadcast multi-access (NBMA) but no neighbor is configured.

2.The OSPF network type is NBMA and a neighbor is configured. However, the broadcast keyword is not specified in a command, such as the map ip command in frame relay (FR). This prevents multicast packets from reaching the peer.

3.The hello intervals and dead intervals of the OSPF neighbors are different.

4.In a stub area or not-so-stubby area (NSSA), some routers are not configured as stub or NSSA routers.
5.The OSPF authentication configuration is incorrect.
6.The OSPF router ID is the same as that of another router.
7.The network types of the two ends of the OSPF link are different.
8.The difference between the maximum transmission units (MTUs) of the two ends of the OSPF link is great.

9.OSPF is disabled on the network.

10.The area numbers are different; the network addresses of the two ends of the link are different. Check the masks of the IP addresses of both ends of the link.

11.The configuration of IP unnumbered is incomplete. Note that when a device is configured with IP unnumbered, the peer must also be configured with IP unnumbered.

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