Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureEasy
A network administrator is troubleshooting slow network performance for users connected to a specific access layer switch. They suspect a Layer 2 loop. Which of the following commands would best help identify the MAC addresses being learned on multiple ports of that switch?
- Ashow cdp neighbors detail
- Bshow ip route
- Cshow interface status
- Dshow mac address-table
Show answer & explanationAnswer & explanation
Correct answer: D. show mac address-table
The 'show mac address-table' command displays the MAC address table, showing which MAC addresses are learned on which interfaces. This is crucial for identifying duplicate MAC addresses on different ports, which is a strong indicator of a Layer 2 loop.
Why the other options are wrong
- A. 'show cdp neighbors detail' displays information about directly connected Cisco devices, not MAC address learning.
- B. 'show ip route' displays the routing table, which is irrelevant for Layer 2 loop detection.
- C. 'show interface status' provides a summary of interface states and configurations, not MAC address learning.
MAC Address Table
A table maintained by a network switch that maps MAC addresses to the specific ports on which they were learned.
- Used by switches to forward frames to specific destinations.
- Entries are learned dynamically when a frame arrives from a source MAC address.
- Can be used to identify Layer 2 loops if the same MAC address appears on multiple non-trunk ports.
Memory trick: Look for the Loop with Learned MACs.