D-Link TM DES-6500, Manual

Page: 159 / 352

DES-6500 Chassis-based Layer 3 Ethernet Switch Manual
158
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.
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.
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router, a router may find that parts of its topological database are out of date.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 6- 120. 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.