350-401 Question #982: Real Exam Question with Answer & Explanation

Question

An engineer receives a report that an application exhibits poor performance. On the switch where the server is connected, this syslog message is visible: SW_MATM-4-MACFLAP_NOTIF: Host 0054.3962.7651 in vlan 14 is flapping between port Gi1/0/1 and port Gi1/0/2. What is causing the problem?

Options

• Aundesirable load-balancing configuration on the switch
• Binvalid port channel configuration on the switch
• Cwrong SFP+ and cable connected between the server and the switch
• Dfailed NIC on the server

Explanation

MAC Flapping & Port Channel Configuration

A MAC address flapping syslog message - where the same MAC address is seen alternating between two different physical ports - is a classic symptom of an incorrectly configured port channel (EtherChannel). When a server's NIC team or bonded interface sends traffic through a port channel, but the switch's EtherChannel is misconfigured (e.g., mismatched LACP/PAgP modes or protocols), the switch cannot bundle the ports logically and instead sees the same MAC appearing on different physical interfaces, causing the flap.

Why the distractors are wrong:

• A (load-balancing): Load-balancing configuration determines how traffic is distributed across a working port channel - it doesn't cause MAC flapping on its own.
• C (wrong SFP+/cable): Physical layer issues would cause link errors or a downed interface, not a flapping MAC address seen on two active ports simultaneously.
• D (failed NIC): A failed NIC would result in a down link or no connectivity, not a MAC alternating between two live ports.

🧠 Memory Tip: Think "flapping between two ports = two ports that should be one (EtherChannel)." MAC flapping across multiple ports almost always points to a port channel/EtherChannel misconfiguration, not a hardware failure.

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