In OSPF, Database Description (DD) packets are used during the database synchronization process. In the first DD packet a router sends during neighbor establishment, three control bits are set:
I bit (Initial) = 1 → This indicates that the DD packet is the first in the sequence.
M bit (More) = 1 → This means more DD packets will follow.
MS bit (Master/Slave) = 1 → Set by the router assuming the master role in the exchange process.
Setting all three bits to 1 is a standard part of the DD exchange initiation.
Reference from Huawei HCIP-Datacom-Core Technology Study Guide:
“In the initial DD packet, the I, M, and MS bits are set to 1 by the master router initiating database exchange.”
(Chapter: OSPF Operation Mechanism – Section: Neighbor State Machine)
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