What is the purpose of the "ip default-gateway" command on a Cisco Layer 2 switch?
Correct Answer: A
Detailed Explanation: This question focuses on the "ip default-gateway" command on Layer 2 switches and its specific purpose. Understanding this command is important for managing Layer 2 switches and enabling remote access for management. Option A accurately describes its function. Layer 2 Switches and Default Gateways: Layer 2 Focus: Layer 2 switches primarily operate at the Data Link Layer. Their main function is to forward frames based on MAC addresses within the same VLAN (broadcast domain). They are typically not involved in IP routing (which is a Layer 3 function). Management VLAN: Layer 2 switches often need to be managed remotely (e.g., via Telnet, SSH, SNMP, web interface). For management purposes, a Layer 2 switch is typically assigned an IP address in a management VLAN. This allows administrators to access the switch management interface from devices on that management VLAN or from networks reachable through that VLAN. Out-of-VLAN Management Access: If you want to manage a Layer 2 switch from a network outside its management VLAN (i.e., out-of-VLAN management), the switch needs a way to reach networks beyond its directly connected VLAN. This is where the "ip default-gateway" command comes in. "ip default-gateway" Command's Purpose: The ip default-gateway command on a Layer 2 switch configures a default route for the switch itself. It tells the switch: "If you need to send IP traffic to a destination network that is not directly connected to any of your VLAN interfaces, send that traffic to this IP address (the default gateway)." This default gateway is typically the IP address of a router interface that is in the same VLAN as the switch's management IP address and can route traffic to other networks. Why Option A is Correct: Default Route for Switch Management: Option A accurately describes the primary purpose: setting a default route for the switch itself. This default route is essential for enabling out-of-VLAN management access to the switch. It allows management traffic originating from networks outside the management VLAN to reach the switch's management IP address and vice versa (switch responses back to management stations). Why Other Options are Incorrect: B. To define the gateway IP address that the switch will assign to DHCP clients in its VLAN. This describes the function of a DHCP server option, specifically option 3 (Router option). DHCP servers provide gateway IP addresses to DHCP clients so clients know where to send traffic destined for networks outside their local subnet. ip default-gateway on a Layer 2 switch does not configure DHCP server options for clients. It's for the switch's own routing, not for client DHCP configuration. C. To enable Layer 3 routing functionality on the switch, turning it into a Layer 3 switch. ip default-gateway on a Layer 2 switch does not turn it into a Layer 3 switch. Layer 3 switches have full routing capabilities (they can participate in routing protocols, maintain routing tables, and route traffic between different networks and VLANs). Layer 2 switches, even with ip default-gateway, primarily remain Layer 2 devices focused on MAC address-based forwarding within VLANs. ip default-gateway only provides a very limited routing capability for the switch's own management traffic, not for general data traffic routing. To make a switch a true Layer 3 switch, you need to enable IP routing (ip routing command) and configure VLAN interfaces (SVIs) with IP addresses to act as Layer 3 interfaces for different VLANs. D. To set the gateway IP address for all devices connected to access ports on the switch. ip default-gateway on a Layer 2 switch does not set the gateway for devices connected to its access ports. The gateway IP address for end devices is typically provided via DHCP (option 3) by a DHCP server or is configured statically on each device. Layer 2 switches are transparent to client IP configuration in terms of setting default gateways for clients. Clients get their gateway information from DHCP servers or static configuration, not from the Layer 2 switch itself. The Layer 2 switch just forwards client traffic within the VLAN based on MAC addresses. In Conclusion: The "ip default-gateway" command on a Cisco Layer 2 switch is specifically for configuring a default route for the switch's own management traffic, primarily to enable out-of-VLAN management access. It does not turn a Layer 2 switch into a Layer 3 switch, nor does it configure default gateways for DHCP clients or end devices connected to access ports. Understanding this command is essential for network management and for distinguishing between Layer 2 and Layer 3 switch functionalities. This question targets a specific but often misunderstood command and its precise application on Layer 2 devices.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.