stp的状态机有哪些？
1.阻塞状态
Discards frames received on the port（不收数据帧）
Discards frames switched from another interface for forwarding（不转发从其它端口接收的数据帧）
Does not learn addresses（不学习MAC地址）
Receives BPDUs（接收BPDU）
2. Listening State
Discards frames received on the port（不收数据帧）
Discards frames switched from another interface for forwarding（不转发从其它端口接收的数据帧）
Does not learn addresses（不学习MAC地址）
Receives BPDUs（接收BPDU）
3. Learning State
Discards frames received on the port（不收数据帧）
Discards frames switched from another interface for forwarding（不转发从其它端口接收的数据帧）
Learns addresses（学习MAC地址）
Receives BPDUs （接收BPDU）
4. Forwarding State
Receives and forwards frames received on the port（接收并且转发数据帧）
Forwards frames switched from another port（转发从其它端口接收的数据帧）
Learns addresses（学习MAC地址）
Receives BPDUs（接收BPDU）
5. Disabled State
Discards frames received on the port（不收数据帧）
Discards frames switched from another interface for forwarding（不转发从其它端口接收的数据帧）
Does not learn addresses（不学习MAC地址）
Does not receive BPDUs（不接收BPDU）
