导致交换机端口呈现err-disable状态的原因有很多，为方便大家查询，特归纳如下：
1. duplex mismatch (A is correct)
2. port-channel misconfiguration
3. Bridge Protocol Data Unit (BPDU) Guard violation
4. UniDirectional Link Detection (UDLD) condition (B is correct)
5. late-collision detection
6. link-flap detection
7. security violation
8. Port Aggregation Protocol (PAgP) flap
9. Layer 2 Tunneling Protocol (L2TP) Guard
10. DHCP snooping rate-limiting
11. EtherChannel guard detects a misconfigured EtherChannel
2. port-channel misconfiguration
3. Bridge Protocol Data Unit (BPDU) Guard violation
4. UniDirectional Link Detection (UDLD) condition (B is correct)
5. late-collision detection
6. link-flap detection
7. security violation
8. Port Aggregation Protocol (PAgP) flap
9. Layer 2 Tunneling Protocol (L2TP) Guard
10. DHCP snooping rate-limiting
11. EtherChannel guard detects a misconfigured EtherChannel
