Hellos perform four functions:
- Discover other OSPF routers n common networks
- Check for agreement of OSPF parameters
- Verify bi-directional visibility between routers
- Monitor health of neighbors to react if the neighbor fails
OSPF routers will send Hello Messages to Multicast address of 126.96.36.199 (All OSPF routers) address.
Hello messages will always source the primary IP of an interface, and never the secondary IP of an interface.
Once two routers discover each other by receiving Hellos, they begin to check for specific parameters seen on the received Hello messages:
- Must pass authentication
- Must be in the same primary subnet
- Must be in the same OSPF area
- Must be of the same area type
- Must no have duplicate RIDs
- OSPF Hello and Dead timers must be equal
If any of these parameters mismatch a neighborship will not be formed. The only thing that doesn’t have to match is the OSPF process ID since that is locally significant to the router.
Hellos are used to bidirectionally verify visibility of the neighbor, the trailing part of each Hello contains the list of all RIDs the originating router has received Hello messages from. If the receiving router sees its own RID in the received Hello message, this confirms bidirectional communication with the neighbor.
Hellos are also responsible for monitoring the ongoing health of the neighborship connection. Hellos are sent every Hello interval and failure to receive Hellos for a period longer than the Dead interval time causes a router to believe that its neighbor has failed.
The Hello interval default time is 10seconds.
The Dead interval is 30 seconds for broadcast networks, and 4xHello interval for NBMA or point to multipoint networks.