xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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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:
Link-State Advertisement Header
Link-State Age
Options Link-State Type
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
Figure 8- 25. 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 Checksum
The Fletcher checksum of the complete contents of the link state
advertisement, 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.
