When Would A Switch Record Multiple Entries For A Single Switch Port In Its Mac Address Table?

A switch may record multiple MAC entries for a single port in its MAC address table when multiple devices are connected to that port through a hub or another switch. This can also happen due to virtualization (multiple VMs using one NIC), port channeling (link aggregation), or MAC address spoofing. Additionally, network loops or misconfigurations may cause the switch to detect multiple MACs on the same port. Proper segmentation or enabling port security can help prevent such issues.

Tech Professionals

07 April 2025

Cisco

When Would A Switch Record Multiple Entries For A Single Switch Port In Its Mac Address Table?

Introduction to CAM Tables

In networking, switches play a crucial role in forwarding data frames efficiently by using their MAC address tables (also known as Content Addressable Memory (CAM) tables). Typically, a switch associates a single MAC address with a single port. However, there are scenarios where a switch may record multiple MAC addresses for a single switch port. Understanding these scenarios is essential for network troubleshooting and certification exams like CCNA (200-301).

This article explores the reasons behind multiple MAC entries on a single switch port, their implications, and how Study4Pass provides the best study materials for mastering such CCNA concepts.

Understanding the MAC Address Table

Before diving into multiple MAC entries, let’s briefly recap how a switch’s MAC address table functions:

Learning: When a switch receives a frame, it records the source MAC address and the incoming port number in its MAC table.
Forwarding: The switch checks the destination MAC address in the frame and forwards it only to the relevant port (if the MAC is known) or floods it out all ports (if unknown).
Aging: MAC entries have a timer (usually 300 seconds) and are removed if no traffic is seen from that MAC address.

Normally, a switch maintains a one-to-one mapping between a MAC address and a port. However, exceptions exist.

Scenarios Where a Switch Records Multiple MAC Addresses for a Single Port

1. Multiple Devices Connected via a Hub

Explanation: If a hub (a legacy networking device) is connected to a switch port, all devices attached to that hub share the same switch port.
Effect: The switch sees multiple MAC addresses arriving through the same port and records them accordingly.
Why It Happens: Hubs operate at Layer 1 (Physical Layer) and do not filter traffic—they simply broadcast all incoming data to every connected device.

2. Virtual Machines (VMs) or Hypervisors

Explanation: A single physical server running multiple virtual machines (VMs) may have each VM configured with its own virtual NIC (vNIC).
Effect: The switch sees different MAC addresses from the same physical port (since the hypervisor manages VM traffic).
Why It Happens: Virtualization platforms like VMware, Hyper-V, or VirtualBox generate unique MAC addresses for each VM.

3. Network Attached Devices (IP Phones, Access Points, etc.)

Explanation: Some devices, such as IP phones or wireless access points (APs), have multiple network interfaces (e.g., a phone may have a PC connected to its passthrough port).
Effect: The switch sees two MAC addresses (one from the phone and one from the attached PC) on the same port.
Why It Happens: These devices act as mini-switches, forwarding traffic from downstream devices.

4. MAC Address Spoofing or Attacks

Explanation: Malicious actors may use MAC flooding attacks to overwhelm a switch’s CAM table by sending frames with randomized source MAC addresses.
Effect: The switch fills its MAC table with fake entries, leading to performance degradation or fail-open mode (acting like a hub).
Why It Happens: Attackers exploit switch behavior to intercept traffic.

5. Port Channel (EtherChannel/LAG) Misconfiguration

Explanation: If EtherChannel (Link Aggregation Group - LAG) is misconfigured, the switch may see the same MAC address on multiple ports.
Effect: The switch may incorrectly associate a MAC with multiple ports, leading to flooding or loops.
Why It Happens: Improper load balancing or inconsistent LACP settings.

Troubleshooting Multiple MACs on a Single Port

If unexpected multiple MAC entries appear on a switch port, network administrators should:

Check for Hubs or Unmanaged Switches – Replace them with proper switches.
Verify Virtualization Configurations – Ensure VMs are correctly managed.
Inspect Connected Devices – Look for IP phones, APs, or other multi-interface devices.
Monitor for MAC Spoofing – Use port security to limit allowed MAC addresses.
Review EtherChannel Settings – Ensure LACP or PAgP is properly configured.

How Study4Pass Helps You Master CCNA Switching Concepts?

For Cisco aspirants, understanding MAC address tables, switching behaviors, and troubleshooting is crucial. Study4Pass provides:

Detailed CCNA (200-301) Study Guides – Covering all exam topics, including Chapter 5 (Switching Concepts).
Real-World Lab Scenarios – Simulating switch behaviors for hands-on learning.
Practice Exams & Quizzes – Testing knowledge on MAC tables, VLANs, and security.
Expert Explanations – Breaking down complex topics into simple, digestible lessons.

By using Study4Pass, you gain the confidence to tackle CCNA exam questions on switch MAC address tables, port security, and network troubleshooting effectively.

Final Thoughts

A switch typically records one MAC address per port, but exceptions occur due to hubs, virtualization, multi-interface devices, attacks, or misconfigurations. Recognizing these scenarios is vital for network troubleshooting and CCNA certification success.

For the best CCNA (200-301) preparation, trust Study4Pass—your ultimate resource for structured learning, practice tests, and expert guidance. Start your journey today and pass your CCNA with flying colors!