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và sau đó sử dụng để chỉ ra các phân đoạn khác nhau trên mạng. Một quản trị viên mạng có thể vay bất kỳ số lượng bit từ phần chủ nhà, miễn là một vài bit còn lại có sẵn để gán địa chỉ host. Chú ý Subnetting mở rộng mạng lưới | elected as master and the other as slave. The master is responsible for sending the DBD packets when either of the following is true When the slave acknowledges the previous DBD packet by echoing the DD sequence number When a set number of seconds configured by the retransmit interval elapses without an acknowledgment in which case the previous DBD packet is retransmitted The slave is not allowed to form the DBD packet. DBD packets are sent in response only to DBD packets received from the master. If the DBD packet received from the master is new a new packet is sent otherwise the previous DBD packet is re-sent. If a situation arises when the master has finished sending the DBD packet and the slave still has packets to send the master sends an empty DBD packet with the M more bit set. The M bit is used to indicate that there are still more packets to send. At this point the master sends an empty DBD packet with the M bit set. Note that when a router receives a DBD packet that contains an MTU field larger than the largest IP datagram the router will reject the packet. Figure 9-4 shows a DBD packet and all the fields in the packet. Figure 9-4. A DBD Packet The following list describes the fields in a DBD packet Interface MTU This is the largest IP datagram that can be sent across the interface without fragmentation. I bit When set to 1 this bit indicates the first packet in the sequence of DBD packets. M bit 198 When set to 1 this bit indicates that more DBD packets are to follow. MS bit This bit indicates the status of the router. When set to 1 the router is the master. When set to 0 the router is the slave. DBD sequence number This indicates the sequence of DBD packets. The initial value should be unique and the value must be incremented until the complete database has been sent. LSA header As the name indicates this field consists of the header of each LSA and describes pieces of the database. If the database is large the entire LSA header cannot fit into a single
