Single connection router-on-a-stick for inter-VLAN routing A switch has three VLANs (VLAN 2, VLAN 3, and VLAN 4). To route between these VLANs using only one physical connection between the switch and the router, what type of Ethernet interface should the router provide to support 802.1Q subinterfaces on that single link?

Introduction / Context:
Inter-VLAN routing can be achieved with a single physical link between a router and a switch using the router-on-a-stick design. The router must support VLAN tagging (IEEE 802.1Q) on that interface, and subinterfaces are configured for each VLAN. The question focuses on which router interface type is appropriate when only one connection is used.

Given Data / Assumptions:

• Three VLANs exist on the switch (2, 3, 4).
• Only one router-to-switch physical connection is allowed.
• 802.1Q trunking will carry multiple VLANs over a single Ethernet link.

Concept / Approach:

A router-on-a-stick requires one Ethernet interface that supports 802.1Q trunking and subinterfaces. Historically, Fast Ethernet (100 Mbps) has been the standard minimum interface speed in classic exam contexts, though Gigabit Ethernet also works. Serial interfaces (e.g., 56 Kbps) do not support Ethernet trunking, and multiple serials would not meet the single-link constraint.

Step-by-Step Solution:

Verification / Alternative check:

Platform guides show that encapsulation dot1Q is configured on a router’s FastEthernet or GigabitEthernet subinterfaces to carry multiple VLANs on a single link.

Why Other Options Are Wrong:

• 10Mbps Ethernet: Technically possible if it supports 802.1Q, but exams typically expect Fast Ethernet as the baseline.
• 56Kbps Serial: Not Ethernet; cannot carry 802.1Q frames.
• 1Gbps Ethernet: Also viable, but the canonical choice in traditional questions is Fast Ethernet.
• Multiport serial: Not relevant to VLAN trunking over Ethernet.

Common Pitfalls:

• Confusing router-on-a-stick (single Ethernet, subinterfaces) with multilayer switching.
• Thinking that a separate physical link per VLAN is required; subinterfaces avoid that.

Final Answer:

100Mbps Ethernet