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

Models: AT-9724TS

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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

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Allied Telesis AT-9724TS Link State Advertisement Header, Router Links Advertisements, Link State ID ASBR,AS External Link