Describe OSPF Packets

There are five types of OSPF packet.

  1. Hello
  2. DBD (DataBase Descriptor)
  3. LSR (Link State Request)
  4. LSU (Link State Update)
  5. LSAck (Link State Acknowledgement)

Hello

Hello is the most common type of packet that is sent. They perform discovery and find the other routers that are available and to maintain neighbor relationships. The default interval for OSPF hellos to be sent are 10 seconds on a LAN interface.

DBD

Database Descriptors are used to send all the route prefixes that a router knows about. It is a summary of the routes that a router has. If there are routes in a received DBD that the local router does not know about then it will send an LSR to get that information from the other router.

LSR

Link State Requests are sent for the specific details of a route that was sent in the DBD. If a router sees a route that it doesn't have then it will send an LSR for the details of that route.

LSU

Link State Update is an answer to an LSR, it is a packet that contains the information that the requesting router requires. The sending router will resend the LSU every 5 seconds if it does not receive an LSAck from the receiving router.

LSAck

Link State Acknowledgement is a confirmation that an LSU packet has been received and processed correctly.

Here is an example of a debug ip ospf packet output and description of the packet information:

Aug 23 08:34:44.043: OSPF rcv. v:2 t:1 l:56 rid:2.2.2.2 aid:0.0.0.0 chk:5728 aut:0 auk: from Ethernet0/0

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