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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
T
ype, 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:
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
(ASBR),AS External Link.
Link State ID This field identifies the portion of the internet environment that is being described by the advertisement.The
contents of this field depend on the advertisement’s Link State Type.
Advertising Router The Router ID of the router that originated the Link State Advertisement. For example,in network links
advertisements this field is set to the Router ID of the network’s Designated Router.
Link State Sequence Number Detects old or duplicate link state advertisements. Successive instances of a link state advertisement are given
successive Link State Sequence numbers.
Link State Check
sum
The Fletcher checksum of the complete contents of the link state adv
er
tisement,including the link state
advertisement header by accepting the Link State Age field.
Length 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:
113
Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Octets
OSPF Link-State Advertisement Header
Link-State ID
Advertising Router
Link-State Checksum Length
Link-State Sequence Number
Link State Age
Options
Link-State Type
01 23
4