2025-02-28 0

Why Does a Switch Show the Same MAC Address on Different Ports?

In a well-managed network, MAC addresses are supposed to be unique identifiers tied to specific devices and switch ports. So, when your switch suddenly reports the same MAC address appearing on multiple ports—say, port Gig0/1 and port Gig0/23—it’s more than a curiosity; it’s a red flag. Duplicate MAC entries can trigger network loops, security breaches, or erratic connectivity. But what causes this anomaly? Is it a misconfiguration, an attack, or faulty hardware? This article uncovers the root causes behind duplicate MAC addresses on switch ports, provides actionable troubleshooting steps, and shares best practices to prevent recurrence.

Understanding MAC Address Learning on Switches

Switches dynamically learn MAC addresses by inspecting the source MAC of incoming frames and mapping them to ports. This information is stored in the MAC address table (or CAM table). Under normal conditions, a MAC address should only appear on one port at a time. When duplicates occur, it signals one of three scenarios:

  1. A Network Loop: A device’s traffic is broadcasted across multiple paths.
  2. MAC Address Conflict: Two devices share the same MAC (intentionally or accidentally).
  3. Security Threats: An attacker spoofs a MAC address to intercept traffic.

Common Causes of Duplicate MAC Addresses

1. Unintentional Network Loops

If Spanning Tree Protocol (STP) is disabled or misconfigured, a looped topology can cause frames to circulate endlessly. The switch sees the same MAC arriving via multiple ports.

Diagnostic Command:

show spanning-tree inconsistentports  # Check for STP failures (Cisco)  
display stp abnormal-port             # Huawei equivalent

2. Virtualization or NIC Teaming

Virtual machines (VMs) or teamed network adapters may clone MAC addresses across hypervisors or physical ports.

Example: Two VMware ESXi hosts using the same MAC for vMotion traffic.

3. Malicious Spoofing Attacks

Attackers use tools like Ettercap to mimic legitimate MAC addresses, redirecting traffic for man-in-the-middle (MITM) attacks.

Red Flags:

  • A printer’s MAC suddenly appears on a server port.
  • Duplicate entries persist even after disconnecting devices.

4. Static MAC Misconfiguration

Manually binding a MAC to multiple ports (e.g., for redundancy) without proper protocols like HSRP/VRRP.

Step-by-Step Troubleshooting

  1. Identify Affected MACs and Ports
    Use the MAC address table to locate duplicates:

    show mac address-table | include aaaa.bbbb.cccc  # Cisco  
display mac-address | grep aaaa-bbbb-cccc        # Huawei
  2. Check for Loops
    • Disable ports one by one to isolate the loop.
    • Verify STP status and ensure it’s active on all VLANs.
  3. Audit Virtualized Environments
    • Ensure hypervisors (VMware, Hyper-V) assign unique MACs to VMs.
    • Disable MAC address replication in NIC teaming configurations.
  4. Hunt for Rogue Devices
    • Compare MACs against an asset inventory.
    • Use port security to block unauthorized MACs: 
      interface Gig0/1  
 switchport port-security  
 switchport port-security mac-address sticky

Configuring MAC Based Authentication on a Switch through the Command Line Interface image024
Figure 1: A Cisco switch displaying the same MAC address on ports Gi0/1 and Gi0/2—a sign of a loop or spoofing.

Permanent Fixes and Prevention

  • Enable STP/RSTP: Prevent loops by activating Spanning Tree Protocol.
  • Implement Port Security: Restrict MAC learning per port (Cisco’s port-security, Huawei’s mac-limit).
  • Use DHCP Snooping + Dynamic ARP Inspection: Block rogue ARP replies in DHCP environments.
  • Audit VM Configurations: Ensure virtualization tools generate unique MACs.

Duplicate MAC addresses on switch ports are rarely harmless. They expose vulnerabilities in network design, security policies, or device management. While temporary fixes like port isolation can restore stability, long-term resilience demands a proactive approach: automate MAC audits, harden port configurations, and educate teams on virtualization pitfalls.

Pro Tip: Leverage network monitoring tools like SolarWinds or PRTG to alert on MAC flapping. For high-security environments, consider MACsec encryption to validate device identities. Remember, a duplicate MAC isn’t just a glitch—it’s a symptom. Diagnose it thoroughly, and your network will thank you.