D-Link DES-3326 - Manual (Page 77)

DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
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with a neighboring router, a router may find that parts of its topological database are out of date.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:
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
Figure 5-29. Link-State Request Packet
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:
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