OSPF Packet Types

There are different OSPF Packet Types which are used for different purposes.

We know that OSPF routers need to perform different duties in order to route traffic. e.g. establish and maintain adjacencies, send and receive requests, ensure reliable delivery of Link State advertisements (LSAs) between neighbors and to describe Link State Databases.

All these duties are performed using messages/Packets. Only one type of message/packet cannot perform all these duties. So, different types of messages/Packets are needed.

OSPF has five types of Packets in total which are listed in below. Each type of packet has its own duties to perform. All these packet types begin with a standard 24 byte header.

1. HELLO: Hello packets are used for neighbor discovery, build neighbor adjacencies and maintain them
2. DBD (Database Descriptor): DBD Packets are a summary of LSDB. They are used to confirm if the LSDB between two routers is same
3. LSR (Link State Request): Request for info from an OSPF neighbor. LSR’s are sent only during Exchange, Loading, or Full state
4. LSU (Link State Update): Reply to LSR (sends the info/packets that were requested from a neighbor)
5. LSAck (Link State Acknowledgement): Acknowledgement Packets

Hello packet (Type1 OSPF Packet), also called Type1 OSPF Packet, is a special packet/message that is sent out periodically from a router to establish and confirm network adjacency relationships. These packets are sent periodically on all interfaces in order to establish and maintain neighbor relationships.

Hello Packets contain different parameters including Network mask, Hello Interval and Router Dead Interval,…. These parameters are needed to form neighbor relationships. All routers connected to a common network (NBMA,…) must agree on certain parameters.

DBD (Database Description / Type2 OSPF Packet) is a sort of summary of the OSPF Database in a router. DBD is used to check if the LSDB between 2 routers is the same.

These packets are exchanged when an adjacency is being initialized. They describe the topology information a router has in its database. DBD uses poll-response procedure (master/slave). Master sends DBD packets/polls which are acknowledged by the slave.

LSR (Link State Request / Type3 OSPF Packets) is simply a request as obvious from its name. After DBD phase for first time, routers keep on sending LSR to keep the LSDB in sync. Similar to LSR, LSU (Link State Update / Type4 OSPF Packet) is the response/reply to LSR. Each Link State Update packet carries a collection of link state advertisements one hop further from its origin.

Finally, Acknowledgements are needed once the LSR/LSU (request/reply) process is complete. Type5 OSPF Packets are used for this purpose which are called LSAck (Link State Acknowledgement / Type5 OSPF Packet).

Below is the detailed Explanation on each type of packet:

OSPF Type1 Message / Hello Packet

OSPF Type2 Message / DBD Packet (Database Descriptor)

OSPF Type3 Message / LSR Packet (Link State Request)

OSPF Type4 Message / LSU Packet (Link State Update)

OSPF Type5 Message / LSAck Packet (Link State Acknowledgment)