Filter
Top Products
DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
152 Switch Management and Operating Concepts
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: