OSPF operational state

OSPF operational state

1. Down state
   • No hello packets received
   • Router sends hello packets
   • Transition to init state
2. Init state
   • Hello packets are received from the neighbor
   • They contain the router ID of the sending router
   • Transition to Two-Way state
3. Two-Way State
   • Communication between 2 routers is bidirectional
   • The router elect DR and BDR
   • Transition to ExStart state
4. ExStart state
   • On the point-to-point networks, the two routers decide which router will initiate DBD packet exchange and decide upon the initial DBD packet sequence number
   • The router with higher router ID will be the first router to send DBD packets
5. ExChange state
   • Routers exchange DBD packets
   • Routers send LSDB information, when the DBD packets are received, it will send LSAck packet to acknowledge.
   • The router use the sequence number to determine the newness of the received link-state information
   • if additional router information is required
     • Transition to Loading state
     • Otherwise, Transition to the FUll state
6. Loading state
   • LSRs and LSUs are used to gain additional route information
   • LSUs are sent only to neighbors in the conditions:
     • when a change is perceived (incremental updates)
     • every 30 minutes
   • Routes are processed using the SPF algorithm
   • Transition to Full state
7. Full state
   • The link-state database of the router is fully synchronized

Establish Neighbor Adjacency

• The hello packets are sent to All OSPF Routers IPv4 multicast address (224.0.0.5)
• The hello packets contain the Router ID
• Only OSPFv2 routers will process these packets
• Router ID is a 32-bit number formatted like an IPv4 address