NETWORKING: OSPF Neighborship

OSPF Neighbor Relationship Process

In this post , We will see about OSPF neighborship process.

First you have to give the IP address to any interface of router which is going to participate in OSPF, As you type router ospf 1,it will choose ospf router id. The criteria is highest IP address will became ospf router id but loopback interface will beat the election process. You can manually configure in router id in ospf process: router-id syntax. This will override every dynamic process.

After that configure network command in ospf routing process for particular interface with proper subnet mask, this will add that interface in ospf routing process, After that particular interface will send hello packets to 224.0.0.5

Have a look on OSPF Packet format:

Hello packet format: (Wireshark view) This is OSPF hello packet captured from tool,You can see the required fields in OSPF Hello packet.

Different States of OSPF neighborship,
There are different states of OSPF neighborship process, Which can be seen by show ip ospf neighbor command.

Down State: In this state no hello packet is received from neighbor within last dead interval.

INIT State: Router A sent Hello packet to router B. Router B received the hello packet but can’t see own router id in hello packet.(the valid hello packet is, receiving router should see its router id in hello in sender’s hello packet)

2 WAY State: This state means in both router bidirectional communication is happened, both router can see each other router id in hello packets. At this state Router decides the how to form the adjacency with other (on broadcast or NBMA) same time it will form full state only with DR and BDR and for other neighbors it will in 2-way state. For point to point link it will form full neighborship. In the stage DR and BDR are elected.

EXSTART State: In this state master and slave negotiation happens between Router and DR/BDR, to start the communication first and exchange the LSA’s. In Point to Point link it negotiation happens between neighbors.

EXCHANGE State: In this state both will exchange DBD (Database descriptors) packets with sequence numbers. Router will send LSR and LSU to each other.'

Loading State: In this state, both routers will compare DBD sent by each other and if some information is missing, Then again router sends LSR and receives LSU. After all process all database is compared, it should be same in both routers. Both should be properly synchronized.

Full State: In this state, routers are fully adjacent with each other. All the router and network LSAs are exchanged and the routers databases are fully synchronized. SPF algorithm is applied and best path is calculated and Routing table of neighbors are completed.

Now we completed the neighborship process. Let's have a look on OSPF packet types:

OSPF Packet Types: