Cisco Certified Network Associate (CCNA) Practice Exam 2025 - Free CCNA Practice Questions and Study Guide

When spanning-tree fails, one significant symptom is that switches experience frequent MAC address table changes. This occurs because, with a malfunctioning spanning-tree protocol, there may be topological loops in the network. In a bridged Ethernet network, such loops can cause frames to circulate indefinitely. As a result, switches continuously see packets from devices, which leads them to update their MAC address tables frequently, thinking devices are moving or connecting/disconnecting. This constant fluctuation can cause network congestion and unpredictable performance.

The other symptoms listed do not directly indicate a spanning-tree failure. For instance, constant traffic across the network, while a potential issue in terms of performance, does not specifically diagnose spanning-tree problems. Routers failing to connect to a VLAN could be caused by various factors, not just spanning-tree issues. Lastly, a lack of configuration changes in devices is not typically associated with spanning-tree failures, as a malfunction might not prevent configuration changes but rather affect how the network behaves.