200-301 Question #1358: Real Exam Question with Answer & Explanation

Question

An Ethernet frame arrived at switch interface G0/1, but the destination MAC address is missing from the MAC address table. How does the switch process the frame?

Options

• AIt sends an ARP request to attempt to locate the destination
• BIt updates the destination to FFFF.FFFF.FFFF.
• CIt drops the frame and notifies the sending host.
• DIt floods the frame out of the remaining switch interfaces.

Explanation

When a switch receives an Ethernet frame for which it does not have the destination MAC address in its MAC address table, it floods the frame out of all interfaces except the one it was received on.

Common mistakes.

• A. ARP (Address Resolution Protocol) operates at Layer 3 to resolve IP addresses to MAC addresses and is initiated by a host or router, not by a Layer 2 switch encountering an unknown MAC address.
• B. The destination MAC address of the frame itself is not updated by the switch to FFFF.FFFF.FFFF; that is a broadcast MAC address which frames can already use as a destination.
• C. A switch typically does not drop unknown unicast frames or notify the sending host unless there's a specific security feature configured (like port security) or if the frame's destination is unreachable after flooding.

Concept tested. Layer 2 switch MAC address table forwarding

Reference. https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/20ew/configuration/guide/sw_layer2.html

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