D-Link DES-3326SRM Product Manual - Page 178

The Link-State Acknowledgment Packet, Link-State Advertisement Formats

Page 178 highlights

D-Link DES-3326S Layer 3 Switch 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 linkstate advertisement headers. The format of the Link-State Acknowledgment packet is shown below: Link-State Acknowledgment Packet Version No. 5 Packet Length Router ID Area ID Checksum Authentication Type Authentication Authentication 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 linkstate 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 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 four types of link state advertisements, each using a common link state header. These are: • Router Links Advertisements • Network Links Advertisements • Summary Link Advertisements • 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 168

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D-Link DES-3326S Layer 3 Switch
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 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 four types of link state advertisements, each using a common link state header.
These are:
Router Links Advertisements
Network Links Advertisements
Summary Link Advertisements
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
168