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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
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Database Description Packet
Version No.
2
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
Checksum
Options
DD Sequence No.
Reserved
Reserved
Link-State Advertisement Header ...
I MMS
Field
Description
Options
The optional capabilities supported by the router.
I – bit
The Initial bit.
When set to 1, this packet is the first in the sequence
of Database Description packets.
M – bit
The More bit.
When set to 1, this indicates that more Database
Description packets will follow.
MS – bit
The Master Slave bit.
When set to 1, this indicates that the router is
the master during the Database Exchange process.
A zero
indicates the opposite.
DD Sequence Number
User to sequence the collection of Database Description Packets.
The initial value (indicated by the Initial bit being set) should be
unique.
The DD sequence number then increments until the
complete database description has been sent.
Database Description Packet
The rest of the packet consists of a list of the topological database’s pieces.
Each link state
advertisement in the database is described by its link state advertisement header.
The Link-State Request Packet
Link-State Request packets are OSPF packet type 3.
After exchanging Database Description
packets with a neighboring router, a router may find that parts of its topological database are
out of date.
The Link-State Request packet is used to request the pieces of the neighbor’s
database that are more up to date.
Multiple Link-State Request packets may need to be used.
The sending of Link-State Request packets is the last step in bringing up an adjacency.
A router that sends a Link-State Request packet has in mind the precise instance of the
database pieces it is requesting, defined by LS sequence number, LS checksum, and LS age,
although these fields are not specified in the Link-State Request packet itself.
The router may
receive even more recent instances in response.
The format of the Link-State Request packet is shown below:
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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
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Version No.
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
Checksum
Link-State Request Packet
3
Link-State Type
Link-State ID
Advertising Router
Each advertisement requested is specified by its Link-State Type, Link-State ID, and
Advertising Router.
This uniquely identifies the advertisement, but not its instance.
Link-
State Request packets are understood to be requests for the most recent instance.
The Link-State Update Packet
Link-State Update packets are OSPF packet type 4.
These packets implement the flooding of
link-state advertisements.
Each Link-State Update packet carries a collection of link-state
advertisements one hop further from its origin.
Several link-state advertisements may be
included in a single packet.
Link-State Update packets are multicast on those physical networks that support
multicast/broadcast.
In order to make the flooding procedure reliable, flooded advertisements
are acknowledged in Link-State Acknowledgment packets.
If retransmission of certain
advertisements is necessary, the retransmitted advertisements are always carried by unicast
Link-State Update packets.
The format of the Link-State Update packet is shown below:
Version No.
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
Checksum
Link-State Update Packet
4
Number of Advertisements
Link-State Advertisements ...
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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
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The body of the Link-State Update packet consists of a list of link-state advertisements.
Each
advertisement begins with a common 20-byte header, the link-state advertisement header.
Otherwise, the format of each of the five types of link-state advertisements is different.
The Link-State Acknowledgment Packet
Link-State Acknowledgment packets are OSPF packet type 5.
To make the folding of link-
state advertisements reliable, flooded advertisements are explicitly acknowledged.
This
acknowledgment is accomplished through the sending and receiving of Link-State
Acknowledgment packets.
Multiple link-state advertisements can be acknowledged in a
single Link-State Acknowledgment packet.
Depending on the state of the sending interface and the source of the advertisements being
acknowledged, a Link-State Acknowledgment packet is sent either to the multicast address
AllSPFRouters, to the multicast address AllDRouters, or as a unicast packet.
The format of this packet is similar to that of the Data Description packet.
The body of both
packets is simply a list of link-state advertisement headers.
The format of the Link-State Acknowledgment packet is shown below:
Version No.
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
Checksum
Link-State Acknowledgment Packet
5
Link-State Advertisement Header ...
Each acknowledged link-state advertisement is described by its link-state advertisement
header.
It contains all the information required to uniquely identify both the advertisement
and the advertisement’s current instance.
Link-State Advertisement Formats
There are five distinct types of link-state advertisements.
Each link-state advertisement
begins with a standard 20-byte link-state advertisement header.
Succeeding sections then
diagram the separate link-state advertisement types.
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
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D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch
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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 five types of link state advertisements, each using a common link state header.
These are:
1. Router Links Advertisements
2. Network Links Advertisements
3. & 4. Summary Link Advertisements
5. 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:
Link-State Advertisement Header
Link-State Age
Options
Link-State Type
Link-State ID
Advertising Router
Link-State Sequence Number
Link-State Checksum
Length
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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:
Type
Description
1
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
excepting the Link State Age field.
Length
The length in bytes of the link state advertisement.
This includes
the 20-byte link state advertisement header.
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:

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