OSPF

1. Hello - Hello packets are used to establish and maintain adjacency with other OSPF routers. The hello protocol is discussed in detail in the next topic.

2. DBD - The Database Description (DBD) packet contains an abbreviated list of the sending router's link-state database and is used by receiving routers to check against the local link-state database.

3. LSR - Receiving routers can then request more information about any entry in the DBD by sending a Link-State Request (LSR).

4. LSU - Link-State Update (LSU) packets are used to reply to LSRs as well as to announce new information. LSUs contain seven different types of Link-State Advertisements (LSAs). LSUs and LSAs are briefly discussed in a later topic.

5. LSAck - When an LSU is received, the router sends a Link-State Acknowledgement (LSAck) to confirm receipt of the LSU.

OSPF router initial procedure

1. Sending Hello packets on all OSPF-enabled interfaces to determine if there are any neighbors on those links. OSPF Hello packets are sent as multicast to an address reserved for ALLSPFRouters at 224.0.0.5 (By default, OSPF Hello packets are sent every 10 seconds on multiaccess and point-to-point segments and every 30 seconds on non-broadcast multiaccess (NBMA) segments (Frame Relay, X.25, ATM). )
   
2. Before two routers can form an OSPF neighbor adjacency, they must agree on three values: Hello interval, Dead interval, and network type.
3. Send Link-state updates (LSUs) to advertise routing information. An LSU packet can contain 11 different types of Link-State Advertisements (LSAs).
•  LSA type: 1    Router LSAs
• type 2             Network LSAs
• type 3 or 4      Summary LSAs
• type 5             Autonomous System Extrenal LSAs
• type 6             Multicast OSPF LSAs
• type 7             Defined for Not-So-Stubby Areas
• type 8             External Attributes LSA for BGP
• type 9,10,11   Opaque LSAs
  
4. Each OSPF router maintains a link-state database containing the LSAs received from all other routers. Once a router has received all of LSAs and built its local link-state database.
   
5. OSPF uses Dijkstra's shortest path first (SPF) algorithm to create an SPF tree. 
6. The SPF tree is then used to populate the IP routing table with the best paths to each network.