Open Shortest Path First (OSPF) uses link-state packets (LSPs) to establish and maintain neighbor adjacencies. It also uses LSPs to exchange routing updates. There are five different OSPF packet types. Beginning of all OSPF packets is specified by standard header of 24 bytes. A list of link state advertisements is deal with all OSPF packet types. The figure below illustrates the five different OSPF packet types. Each link-state packet (LSPs) serves a specific purpose in the OSPF routing process:
Hello packet– This packet establishes and maintains adjacency with other Open Shortest Path First (OSPF) routers. The Open Shortest Path First (OSPF) protocol sends these packet periodically on all interfaces. This is a multicast packet discovering neighboring routers dynamically.
Database Description (DBD) packet– This packet has an abbreviated list of the sending router’s Link-state Database (LSDBs). The receiving routers used DBD to check against the local Link-state Database (LSDBs). The Link-state Database (LSDBs); must be equal on all link-state routers within an area to build a perfect SPF tree. The database is designed from multiple packets.
Link-State Request (LSR) packet – When receiving routers read the DBD then receiving router can request more information about any entry in the DBD, sending a LSR packet. The router uses LSR packets for requesting the pieces of the neighbor’s database which are more up to date and complete. The router can also send multiple Link State Request packets.
Link-State Update (LSU) packet – This packet is used to reply link-state request (LRS) packet. The link-state update (LSU) contain further seven types of packets.
Link-State Acknowledgment (LSAck) packet – When router receive an LSU packet, it sends an LSAck packet to the sender router to confirm receipt of the LSU packet. The LSAck data field is empty. Single Link State Acknowledgement packet can acknowledge the multiple link state advertisements.