Cisco: Factors that fail to establish OSPF Neighbor Relationship

• The Hello interval and dead time at both ends do not match. You can use show ip ospf interface XX to view details. Usually 10/40 s or 30/120 s.
  
  

• The masks at both ends are inconsistent. OSPF neighbor creation requires that the two ports have the same mask.
  
  

• The network types at both ends are inconsistent. The network types include point-to-point, non-broadcast multiaccess, broadcast multiaccess, and point-to-multipoint. You can view the network type of the port through show ip ospf interface. In Port mode, you can use the ip ospf network XX command to modify the port network type.
  
  

• The area-ID at both ends is inconsistent. You must be in the same area to establish a neighbor relationship.
  
  

• The MTU at both ends is inconsistent. You can run the show interface XX command to view the MTU value of the port, usually 1500. If you need to modify it, you can enter port mode and use the mtu xx command to modify it.
  
  

• Special Region ID. If a vro in the same region belongs to a special region, all the vrouters must be configured as special regions. Otherwise, the neighbor relationship cannot be established normally. Special areas include stub area, totally stub area, not-so-stubby-area, and totally NSSA.
  
  

• The router-ID is unique. The router-ID must be unique across the network.

• Authentication is enabled at both ends of the port. If the two sides of the authentication do not match, a neighbor relationship cannot be established.

This article from the "Wine sword Rivers and Lakes" blog, please be sure to keep this source http://wucl202000.blog.51cto.com/4687508/1431489