A network administrator is reviewing switch console logs and port statistics following an intermittent layer 2 outage. Match each observed diagnostic log or symptom on the left to its underlying root cause on the right.
- Interface GigabitEthernet0/1 transitioned to err-disabled state following a BPDUGuard event.A Spanning Tree Protocol BPDU frame was received on a PortFast-enabled edge interface.
- Switch console logs display %CDP-4-NATIVE_VLAN_MISMATCH on trunk port GigabitEthernet0/24.The untagged VLAN ID configured on one end of the trunk link does not match the opposing end.
- Port statistics on a FastEthernet link report high rates of late collisions and FCS errors.One side of the Ethernet link is operating in full-duplex mode while the opposing side operates in half-duplex.
- Traffic for VLAN 40 fails across an 802.1Q trunk while traffic for VLANs 10 and 20 flows normally.The trunk interface configuration has an allowed VLAN list that explicitly omits VLAN 40.
Answer
The diagnostic symptoms correctly match their respective root causes: BPDU Guard err-disable pairs with receiving a BPDU on a PortFast edge port; CDP native VLAN mismatch pairs with conflicting untagged VLAN IDs across the trunk; late collisions pair with a duplex mismatch between link peers; and VLAN 40 traffic dropping across the trunk pairs with exclusion from the trunk allowed VLAN list.
Each diagnostic log or symptom corresponds to a fundamental Layer 2 switching mechanism: BPDU Guard err-disables ports receiving unauthorized BPDUs; CDP native VLAN mismatch alerts identify inconsistent untagged VLAN IDs; late collisions demonstrate duplex setting mismatches; and selective trunk VLAN drops indicate restrictive allowed VLAN list configurations.
Step-by-Step Solution
Key Concept
Troubleshooting Layer 2 Switching, VLAN, and Trunking Failures