D-Link xStack DES-3800 Series User Manual page 142

The body of the Link-State Update packet consists of a list of link-state advertisements. Each advertisement begins with a
common 20-byte header, the link-state advertisement header. Otherwise, the format of each of the five types of link-state
advertisements is different.
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:
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
•
xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
Link-State Acknowledgment Packet
Version No.
Checksum
Link-State Advertisement Header ...
Figure 8- 23. Link State Acknowledge Packet
Packet Length
5
Router ID
Area ID
Authentication Type
Authentication
Authentication
127