3Com Switch 4800G 24-Port Configuration Manual page 285

Figure 83 DD packet format
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Interface MTU
Major fields:
Interface MTU: Size in bytes of the largest IP datagram that can be sent out the
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associated interface, without fragmentation.
I (Initial) The Init bit, which is set to 1 if the packet is the first packet of
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database description packets, and set to 0 if not.
M (More): The More bit, which is set to 0 if the packet is the last packet of DD
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packets, and set to 1 if more DD Packets are to follow.
MS (Master/Slave): The Master/Slave bit. When set to 1, it indicates that the
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router is the master during the database exchange process. Otherwise, the
router is the slave.
DD Sequence Number: Used to sequence the collection of database description
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packets for ensuring reliability and intactness of DD packets between the
master and slave. The initial value is set by the master. The DD sequence
number then increments until the complete database description has been
sent.
LSR packet
After exchanging DD packets, any two routers know which LSAs of the peer
routers are missing from the local LSDBs. In this case, they send LSR (link state
request) packets, requesting the missing LSAs. The packets contain the digests of
the missing LSAs. The following figure shows the LSR packet format.
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2
Router ID
Area ID
Authentication
Authentication
Options
DD sequence number
LSA header
LSA header
Introduction to OSPF 285
Packet length
AuType
0 0 0 0 0 I M
31
M
S