OSPF Hello's
As a CCNA / CCNP candidate you are expected to understand the purpose and function of the OSPF protocol.
The CCNA / CCNP’s exam will ask of you to determine the purpose of the OSPF hello packet, what it contains and the role it plays
Inside the OSPF Hello Packet
OSPF routers create neighbor relationships with directly connected routers by exchanging hello messages. OSPF Router exchange hellos addressed to the ALLSPFROUTERS multicast destination address of 224.0.0.5.
OSPF sends out hello packets every 10 seconds with a hold time of 40secs on a broadcast link or Point-to-point link
OSPF sends out hello packets every 30 seconds with a hold time of 120secs on a non-broadcast link
The Hello Packet contains the following data:
- ROUTER ID = Router ID, this is a 32-bit number that is unique to this router
- *HELLOS & DEAD INTERVALS = Hello and dead interval Period of time between hellos and the dead timer, these values must match between two routers becoming neighbors
- NEIGHBOUR LIST = Is a List of neighbor Router Ids that this router sees as their neighbor
- *AREA ID = Area ID Area number, this value must be identical between two routers becoming neighbors
- ROUTER PRIORIY = Router Priority of this interface, the router advertising the highest priority is elected the designated router during the DR/DBR election process.
- ROUTER ID = The Router ID of who this router believes is the DR and BDR IP address of designated routers
- *AUTHENTICATION = If you have authentication set it is important that the Password matches between the two routers becoming neighbors.
- *STUB AREA FLAG = If the routers becoming neighbors are in a Stub Area, then the Stub Area Flag must be set
Two routers will initially exchange hello and examine the contents of the Hello Packets to verify that the fields which need to match do indeed match.
If any of the fields do not match the routers cannot form any neighbor adjacency, to view the reasons for any possible issues run the following command:
R1#debug ip ospf adj
If you see your neighbour adjacency like below, where the state is FULL means the adjacency has formed:
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.1.3 1 FULL/DR 00:00:39 192.168.1.3 Ethernet0/0
If you see your neighbour adjacency like below, where the state is 2-way means the neighbour relationship has formed and that the neighbour is neither the DR nor the BDR:
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.1.3 1 2-WAY/DROTHER 00:00:39 192.168.1.3 Ethernet0/0
If you see your neighbour adjacency like below, where the state is Exstart the neighbors are past the 2-way state and are determining which device is going to start the exchange of the summary of the Link state database. If the state continues to display exstart can mean that the Interface MTU sizes are mismatched. Run the “debug ip ospf adj” command to show any neighbour formation issues.
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.1.3 1 Exstart/DR 00:00:39 192.168.1.3 Ethernet0/0
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