The router ID must be available before configuring OSPF. The router ID is a 32-bit number assigned to each router running OSPF. This number uniquely identifies the router within an AS. OSPF routers use the router IDs of the neighbor routers to establish adjacencies. Neighbor IDs are learned when Hello packets are received from the neighbor.
Before configuring OSPF parameters, ensure that the router ID is derived by one of the following methods.
Define the value in the config>router router-id context.
Define the system interface in the config>router>interface ip-int-name context (used if the router ID is not specified in the config>router router-id context).
A system interface must have an IP address with a 32-bit subnet mask. The system interface is used as the router identifier by higher-level protocols such as OSPF and IS-IS. The system interface is assigned during the primary router configuration process when the interface is created in the logical IP interface context.
If you do not specify a router ID, then the last four bytes of the MAC address are used.