OSPF packet types
OSPF uses five message types to operate. These are:
Link state request
Link state update
Link state acknowledgment
OSPF message types should not be confused with OSPF LSA Types.*
All OSPF messages are similar in structure. They all contain an OSPF header that contains information about the OSPF version number, a field that indicates the type of message, the area ID, and the source OSPF router of the message.
The rest of the sections of an OSPF packet depend on the type:
- A hello packet will have a hello header, and an OSPF LLS data block
- A database description will have an OSPF DB description header and an LLS data block
- A link-state request will have only a Link State Request header
- A link-state update will have only an LS update header (which contains the LSAs)
- A link-state acknowledgment will have acknowledgments for specific LSAs that were received
The OSPF Link Local Signaling (LLS) data block is an extension of existing OSPF packets in order to provide additional bit space. The additional bit space enables greater information per packet exchange between OSPF neighbors.
You can take a detailed look at what each of these OSPF message types look like in the following Cloudshark packet capture: