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Figure 6- 102.Database Description Packet
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
adv
ertisement header.
Link-State Request Packet
Link-State Request pack
ets 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topological database are out of date.The Link-State Request packet is used to request the pieces of the neighbor’s database that are more up to date.Multiple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 f
ormat of the Link-State Request pack
et is sho
wn belo
w:
110
Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Version No.
2
Packet Length
Octets
01 23
OSPF Database Description Packet
Router ID
Area ID
Checksum Authentication Type
Authentication
Authentication
DD Sequence No.
Link-State Advertisement Header
Reserved I M MS Reserved Options
4