| 33.3. Types of OSPF packets | ||
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 | Chapter 33. OSPF - Open Shortest Path First |  |
| 33.3. Types of OSPF packets | ||
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| | Chapter 33. OSPF - Open Shortest Path First | |
The Open Shortest Path First (OSPF) protocol runs directly over IP, using IP protocol 89. It does not provide any explicit fragmentation/reassembly support.
When fragmentation is necessary, IP fragmentation/reassembly is used. OSPF protocol packets have been designed so that large protocol packets can generally be split into several smaller protocol packets.
This practice is recommended; IP fragmentation should be avoided whenever possible.
OSPF protocol uses the types of packets that are described below:
HELLO | The Hello packet (Hello) | These packets are sent periodically on all interfaces
(including virtual links) in order to establish and maintain
neighbor relationships. In addition, Hello Packets are multicast on those physical networks having a multicast or broadcast capability, enabling dynamic discovery of neighboring routers. |
DD | The Database Description packet | These packets are exchanged when an adjacency is being initialized. They describe the contents of the topological database. Multiple packets may be used to describe the database. For this purpose, a poll-response procedure is used. |
LSR | The Link State Request packet | After exchanging Database Description packets with a neighboring router, a router may find that parts of its topological database are out of date. The Link State Request packet is used to request the pieces of the neighbor's database that are more up to date. Multiple Link State Request packets may need to be used. The sending of Link State Request packets is the last step in bringing up an adjacency. |
LSU | The Link State Update packet | These packets implement the flooding of link state
advertisements. Each Link State Update packet carries a collection
of link state advertisements one hop further from its origin.
Several link state advertisements may be included in a single
packet. Link State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the flooding procedure reliable, flooded advertisements are acknowledged in Link State Acknowledgment packets. If retransmission of certain advertisements is necessary, the retransmitted advertisements are always carried by unicast Link State Update packets. |
LSA | The Link State Acknowledgment packet | To make the flooding of link state advertisements reliable, flooded advertisements are explicitly acknowledged. This acknowledgment is accomplished through the sending and receiving of Link State Acknowledgment packets. Multiple link state advertisements can be acknowledged in a single Link State Acknowledgment packet. |
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| 33.2. OSPF Terminology |  | 33.4. OSPF resource |