OSPF neighbor relationship
Routers on the same network segment are called neighbors. Two routers connected with each other become neighbors if they have the same area-id, subnet, authentication and hello/dead intervals.
Neighbor process starts with hello packets. Hello packets are sent periodically out of each interface using IP multicast. Routers become neighbors as soon as they see themselves listed in the neighbor's Hello packet. This way, a two way communication is guaranteed. This process involves the following steps and states:
1. First router determined OSPF RID
2. Network command add OSPF interfaces into OSPF process and router run link-state
3. Hello packets are send out on OSPF running interfaces.
When OSPF router sends a hello message it is in down-state.
Down-state:
When router sends a hello packet it is down-state, hello message is send in 10s in broadcast / point to point network and after every 30s in NBMA (non broadcast multi-access)
Init-state:
Received Hello from other routers, in this state router check hello timer, check network ID, area ID and authentication.
Two-way State:
If already neighbor then update dead timer, if no then add as a new neighbor and move to next step.
Exstart State:
Master slave are selected in this state. A router having high priority will become Master if they have same then higher RID break the tie. Master sends the information first to slave.
Exchange State:
Routers will describe their entire link-state database by sending database description packets. At this state, packets could be flooded to other interfaces on the router.
Loading State:
In loading state router start save the information in memory. Slave send LSR (link state request), master reply with LSU (link state update), similarly master request for missing info by LSR and slave rely with LSU. By this process neighbors get synchronize.
Full State:
In full state neighbors are synchronized, SPF algorithm is run for calculation shortest path.
OSPF Lab Examples:
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