D-Link DES-3326SR Manual page 177

The Link-State Acknowledgment Packet
Link-State Acknowledgment packets are OSPF packet type 5. To make the folding 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.
Depending on the state of the sending interface and the source of the advertisements being acknowledged, a
Link-State Acknowledgment packet is sent either to the multicast address AllSPFRouters, to the multicast
address AllDRouters, or as a unicast packet.
The format of this packet is similar to that of the Data Description packet. The body of both packets is simply a
list of link-state advertisement headers.
The format of the Link-State Acknowledgment packet is shown below:
Version No.
Each acknowledged link-state advertisement is described by its link-state advertisement header. It contains all
the information required to uniquely identify both the advertisement and the advertisement's current instance.
Link-State Advertisement Formats
There are five distinct types of link-state advertisements. Each link-state advertisement begins with a standard
20-byte link-state advertisement header. Succeeding sections then diagram the separate link-state advertisement
types.
Each link-state advertisement describes a piece of the OSPF routing domain. Every router originates a router
links advertisement. In addition, whenever the router is elected as the Designated Router, it originates a network
links advertisement. Other types of link-state advertisements may also be originated. The flooding algorithm is
reliable, ensuring that all routers have the same collection of link-state advertisements. The collection of
advertisements is called the link-state (or topological) database.
From the link-state database, each router constructs a shortest path tree with itself as root. This yields a routing
table.
There are four types of link state advertisements, each using a common link state header. These are:
•
Router Links Advertisements
•
Network Links Advertisements
•
Summary Link Advertisements
•
Autonomous System Link Advertisements
D-Link DES-3326SR Layer 3 Switch
Link-State Acknowledgment Packet
5
Checksum
Link-State Advertisement Header ...
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
167