3Com MSR 50 Series Configuration Manual page 929

Figure 279 DD packet format
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Interface MTU
Major fields:
Interface MTU: The size in bytes of the largest IP datagram that can be sent out
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the associated interface, without fragmentation.
I (Initial) The Init bit, which is set to 1 if the packet is the first packet in the
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sequence of 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 in the
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sequence of DD 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
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Description 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. Figure 280
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Router ID
Area ID
Authentication
Authentication
Options
DD sequence number
LSA header
LSA header
shows the LSR packet format.
Introduction to OSPF 929
Packet length
AuType
0 0 0 0 0 I M
31
M
S