The OSPF packet types
- Hello packet – These packets are multicast periodically to 224.0.0.5 multicast address on all interfaces (unicast on virtual-links) enabling dynamic discovery of neighbours and maintain neighbour relationships. On broadcast and NBMA networks, Hello packets are used to elect DR and BDR.
The following hello packets must match on neighboring routers to establish adjacency:
- Hello interval
- Dead interval
- Area ID
- Authentication password
- Stub Area Flag
- Not in hello packet but also must match – MTU, ip addresses on interfaces must be on same subnet with same subnet mask
- Database Descriptor packet – For link-state routing protocol, it is required that the link-state databases for all routers remain synchronized. The synchronization starts as soon as the adjacency is formed between neighbours. OSPF uses Database Descriptor (DBD) packets for this purpose.These packets are exchanged when adjacency is being initialized. They describe the content of the LSDB. Multiple DBD packets may be exchanged to describe the database.
- Link State Request packet – After DBD packets exchange process, the router may find it does not have an up-to-date database. The LSR packet is used to request pieces of a neighbour database that are more up-to-date.
- Link State Update packet – Link State Update (LSU) packets are OSPF packet Type 4. These packets implement the flooding of LSAs. Each LSA contains routing, metric and topology information to describe a portion of OSPF network. The local router advertises LSA within an LSU packet to its neighbouring routers. In addition, the local router advertises the LSU packet with information in response to an LSR packet.
- Link State Acknowledgement packet – OSPF requires an acknowledgement for the receipt of each LSA. Multiple LSAs can be acknowledged in a single LSAck packet.