Allied Telesis AT-9724TS Link State Advertisement Header, Router Links 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:

OSPF Link-State Advertisement Header
Octets
0	1	2		3	4
	Link State Age		Options		Link-State Type
		Link-State ID

Advertising Router

Link-State Sequence Number

Link-State Checksum

Length

	Figure 6- 106. Link State Advertisement Header
Field	Description
Link State Age	The time is seconds since the link state advertisement was originated.
Options	The optional capabilities supported by the described portion of the routing domain.
Link State Type	The type of the link state advertisement. Each link state type has a separate advertisement format.
	The link state type are as follows: Router Links, Network Links, Summary Link (IP Network), Summary Link
Link State ID	(ASBR),AS External Link.
	This field identifies the portion of the internet environment that is being described by the advertisement.The
Advertising Router	contents of this field depend on the advertisement’s Link State Type.
	The Router ID of the router that originated the Link State Advertisement. For example, in network links
Link State Sequence Number	advertisements this field is set to the Router ID of the network’s Designated Router.
	Detects old or duplicate link state advertisements. Successive instances of a link state advertisement are given
Link State Checksum	successive Link State Sequence numbers.
	The Fletcher checksum of the complete contents of the link state advertisement, including the link state
Length	advertisement header by accepting the Link State Age field.
	The length in bytes of the link state advertisement.This includes the 20-byte link state advertisement header.

Router Links Advertisements

Router links advertisements are type 1 link state advertisements. Each router in an area originates a routers links advertisement.The advertisement describes the state and cost of the router’s links to the area.All of the router’s links to the area must be described in a single router links advertisement.

The format of the Router Links Advertisement is shown below:

Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch 113