DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
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
The 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:
152 | Switch Management and Operating Concepts |