D-Link xStack DES-3800 Series User Manual page 141

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router may find that parts of its topological database are out of date.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:
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.
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:
xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
Link-State Request Packet
Version No.
Checksum
Figure 8- 21. Link-State Request Packet
Link-State Update Packet
4
Version No.
Checksum
Figure 8- 22. Link-State Update Packet
3
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
Link-State Type
Link-State ID
Advertising Router
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
Number of Advertisements
Link-State Advertisements ...
126