D-Link DGS-3312SR Product Manual page 117

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
Link State Advertisement Header
All link state advertisements begin with a common 20-byte header. This header contains enough information to uniquely
identify the advertisements (Link State Type, Link State ID, and Advertising Router). Multiple instances of the link state
advertisement may exist in the routing domain at the same time. It is then necessary to determine which instance is more
recent. This is accomplished by examining the link state age, link state sequence number and link state checksum fields
that are also contained in the link state advertisement header.
The format of the Link State Advertisement Header is shown below:
DGS-3312SR Gigabit Layer 3 Switch
Link-State Acknowledgment Packet
Version No.
Checksum
Link-State Advertisement Header ...
Figure 5- 13. Link-State Acknowledgment Packet
Packet Length
5
Router ID
Area ID
Authentication Type
Authentication
Authentication
104