Configuring a trunk on a Cisco Catalyst 2960: Which command correctly configures a switchport as an IEEE 802.1Q trunk on a 2960 switch?

Introduction / Context:
On Cisco Catalyst 2960 switches, enabling a trunk port is a frequent task to carry multiple VLANs between switches or between a switch and a router-on-a-stick. Understanding the exact command syntax prevents misconfiguration.

Given Data / Assumptions:

• Platform: Catalyst 2960.
• Goal: Configure a port to operate as a VLAN trunk using 802.1Q.

Concept / Approach:
The 2960 supports only 802.1Q encapsulation and does not allow changing trunk encapsulation type. The correct approach is to enter interface configuration mode and set the switchport to trunk mode.

Step-by-Step Solution:

Verification / Alternative check:
Use show interfaces trunk to verify the interface is in trunking mode and carrying expected VLANs.

Why Other Options Are Wrong:

• trunk on / trunk all: Not valid IOS commands.
• switchport trunk on: Not valid syntax; the correct form uses “mode.”
• switchport encapsulation dot1q: On 2960, encapsulation is fixed to 802.1Q; this command is not available on that platform.

Common Pitfalls:
Trying to set encapsulation on platforms that do not support ISL; forgetting to allow specific VLANs if the default allowed list was modified elsewhere.

Final Answer:
switchport mode trunk