Default VLANs permitted on a new 802.1Q trunk On an IOS-based switch, after creating a new 802.1Q trunk between switches, which VLANs are permitted across that trunk by default (before any pruning or allowed-vlan filtering is configured)?

Introduction / Context:
When enabling an 802.1Q trunk on Cisco IOS switches, the default behavior determines which VLANs can traverse the link. Understanding the default allowed VLAN set helps avoid connectivity surprises and informs whether additional pruning or explicit switchport trunk allowed vlan commands are required.

Given Data / Assumptions:

• Two Cisco IOS switches are connected via an 802.1Q trunk.
• No explicit allowed-vlan list or pruning has been configured yet.
• The question asks for the default behavior.

Concept / Approach:

By default, Cisco switches allow all VLANs across a newly formed trunk. Administrators can later restrict this with allowed-vlan lists or enable VTP pruning to limit unnecessary VLAN traffic. This default simplifies initial connectivity but may carry unneeded broadcasts if left unrestricted.

Step-by-Step Solution:

Verification / Alternative check:

Use show interfaces trunk to see the allowed and active VLAN lists and confirm current behavior before and after changes.

Why Other Options Are Wrong:

• Manual-only permission (B) is false; defaults allow all.
• Only configured VLANs (C) or only extended VLANs (D) are not the default behavior.
• Only VLAN 1 (E) is incorrect; although VLAN 1 is native by default, all VLANs are allowed unless restricted.

Common Pitfalls:

• Leaving trunks unrestricted can spread broadcasts and security domains unnecessarily.
• Confusing the native VLAN concept (untagged) with the allowed VLAN list.

Final Answer:

By default, all VLANs are allowed on the trunk.