OSPF Router Communication

OSPF runs directly on IPv4, using it’s own protocol numbered 89, reserved for OSPF by IANA (Internet Assigned Numbers Authority).

OSPF uses multicast where possible to reduce unnecessary broadcast or unicast traffic:

  • Multicast address or MAC 01:00:5E:00:00:05. All Routers using OSPF receive these packets.
  • Multicast address or MAC 01:00:5E:00:00:06. Only designated routers receive this traffic.
1HelloUsed for discovering and maintaining neighbours. Sent out periodically.
2Database Description (DBD/DDP)Used for summarising database contents. Packets are exchanged on the first formation of the OSPF adjacency. Used to describe the contents within the LSDB
3Link-state request (LSR)Used for database downloads. When a router wants to update part of it’s LSDB, it will require this part of the LSDB from a neighbour using this packet type.
4Link-state update (LSU)Used for database updates. This is a specific LSA for a network link and is normally sent in response to a Type 3 / LSR packet
5Link-state ackUsed in response to the flooding of LSAs. The response allows the packet flooding to be reliable.
OSPF Packet Types



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