How do you verify and reset a trunk configuration?

Use show interfaces switchport to verify trunk settings. Reset to defaults with no switchport trunk allowed vlan and no switchport trunk native vlan.

Verification with show interfaces fa0/1 switchport:

Key fields to check:

• Administrative Mode: trunk — configured as trunk
• Operational Mode: trunk — actually functioning as trunk
• Administrative Trunking Encapsulation: dot1q — using 802.1Q
• Trunking Native Mode VLAN: 99 (VLAN0099) — native VLAN is 99
• Trunking VLANs Enabled: ALL — which VLANs are allowed

Resetting trunk to defaults:

S1(config)# interface fa0/1
S1(config-if)# no switchport trunk allowed vlan
S1(config-if)# no switchport trunk native vlan
S1(config-if)# end

This resets:

• Allowed VLANs (Virtual Local Area Networks) → ALL (1–4095)
• Native VLAN → VLAN 1 (default)

Converting trunk back to access:

S1(config-if)# switchport mode access

This changes both the administrative and operational mode to static access, and the port will belong to its assigned access VLAN (or VLAN 1 by default).