A switch is configured with three VLANs (VLAN2, VLAN3, and VLAN4). A router has been added to provide inter VLAN communication, and only a single physical connection is planned between the router and the switch. What is the minimum type of interface required on the router to support this one connection for routing between all three VLANs (router on a stick)?

Introduction / Context:
When multiple VLANs exist on a switch and a router is used to provide inter VLAN routing, a common design is the router on a stick model. In this design, a single physical interface on the router is configured as a trunk to the switch and multiple subinterfaces are created, one per VLAN. Each subinterface has an IP address in the corresponding VLAN subnet and acts as the default gateway for that VLAN. This question asks which type of router interface is the minimum appropriate choice for this trunk connection in a typical modern network.

Given Data / Assumptions:

• The switch has three VLANs: VLAN2, VLAN3, and VLAN4.
• Only one physical link will connect the router and the switch.
• The router will handle inter VLAN routing using subinterfaces over a trunk link.
• The available interface types include low speed serial and various Ethernet speeds.

Concept / Approach:
Router on a stick requires an Ethernet interface that supports trunking with IEEE 802.1Q or ISL encapsulation. A serial interface is not appropriate for an 802.1Q trunk to a switch, and 56 Kbps is far too slow for typical LAN inter VLAN routing needs. While a 10 Mbps Ethernet port could technically carry VLAN tagged traffic, many exam style questions assume Fast Ethernet as the minimum practical standard in switched LAN environments. Fast Ethernet provides 100 Mbps bandwidth and can be configured for trunking, making it the usual choice for inter VLAN connections in such scenarios, unless higher speed requirements dictate Gigabit Ethernet.

Step-by-Step Solution:
1. Recognize that inter VLAN routing through a single physical link requires an Ethernet trunk between the router and switch. 2. Understand that this is implemented by creating multiple subinterfaces on the router's Ethernet port, each configured with encapsulation dot1q and an IP address for its VLAN. 3. Eliminate serial interfaces because they cannot serve as 802.1Q trunks in a standard router on a stick design to a LAN switch. 4. Evaluate 10 Mbps Ethernet versus 100 Mbps Fast Ethernet. Both can technically support trunking but 100 Mbps is typically considered the minimum recommended bandwidth in modern campus designs and matches exam expectations. 5. Note that 1 Gbps Gigabit Ethernet is an even better choice but is not strictly the minimum required to make the design work. 6. Conclude that the minimum appropriate interface type is a 100 Mbps Fast Ethernet interface.

Verification / Alternative check:
Exam questions from Cisco certified training material frequently describe router on a stick using a Fast Ethernet interface on the router, configured with subinterfaces like Fa0.1, Fa0.2, and Fa0.3 for VLAN2, VLAN3, and VLAN4. The corresponding switch port is configured as a trunk. This consistent use of Fast Ethernet indicates that it is considered the baseline interface type for this design in test scenarios, confirming the choice of a 100 Mbps Fast Ethernet interface as the correct answer.

Why Other Options Are Wrong:
Option a: A 10 Mbps Ethernet interface could technically work, but it is usually not presented as the minimum recommended option in certification contexts where Fast Ethernet is expected. Option b: A 56 Kbps serial interface cannot function as an 802.1Q Ethernet trunk and also lacks the bandwidth required for normal LAN inter VLAN traffic. Option d: A 1 Gbps Gigabit Ethernet interface is more than sufficient but does not represent the minimum type of interface required in this question. Option e: Serial interfaces with Frame Relay encapsulation are used for WAN connections and are not appropriate for Ethernet trunking to a LAN switch.

Common Pitfalls:
One common mistake is to think that any router interface type is acceptable as long as IP can be configured. However, router on a stick relies on Ethernet trunking, which requires an Ethernet interface that supports VLAN tagging. Another pitfall is focusing on maximum performance and choosing Gigabit Ethernet by default, even when the question specifically asks for the minimum interface type. Reading the wording carefully and remembering typical LAN speeds used in exam examples will help you choose the most appropriate answer in context.

Final Answer:
The minimum interface type needed on the router for a single trunk link that carries multiple VLANs is a 100 Mbps Fast Ethernet interface.