Which Information Does A Switch Use To Populate The MAC Address Table?

Introduction

In computer networking, switches play a crucial role in forwarding data frames efficiently within a local area network (LAN). One of the key functions of a switch is to maintain a MAC (Media Access Control) address table, also known as a CAM (Content Addressable Memory) table. This table helps the switch make intelligent forwarding decisions by mapping MAC addresses to the corresponding switch ports.

For students preparing for the Cisco CCNA 200-301 exam or the CCNA 2 Pretest (V5.03, V5.1, v6.0), understanding how a switch populates its MAC address table is essential. This article provides a detailed explanation of the process, along with exam-focused insights.

Understanding MAC Addresses and Switching

What is a MAC Address?

A MAC address is a 48-bit (6-byte) unique identifier assigned to a network interface card (NIC) by the manufacturer. It is written in hexadecimal format (e.g., 00:1A:2B:3C:4D:5E).

How Does a Switch Work?

Unlike hubs, which broadcast data to all ports, a switch operates at Layer 2 (Data Link Layer) and forwards frames only to the intended destination port. To do this efficiently, the switch maintains a MAC address table.

How a Switch Learns MAC Addresses?

A switch dynamically learns MAC addresses by examining the source MAC address of incoming frames. Here’s how the process works:

1. Frame Reception: When a device sends a data frame, the switch receives it on a specific port.
2. Source MAC Extraction: The switch reads the source MAC address from the frame header.
3. MAC-Port Mapping: The switch adds the source MAC address and the incoming port number to its MAC address table.
4. Forwarding Decision: If the destination MAC address is already in the table, the switch forwards the frame only to the relevant port. If not, it floods the frame to all ports (except the incoming port).

The MAC Address Table (CAM Table)

The MAC address table (or CAM table) is a database that stores:

• MAC Address – The physical address of the connected device.
• Port Number – The switch port where the device is connected.
• VLAN ID – The Virtual LAN associated with the port (if VLANs are configured).
• Aging Timer – A countdown before the entry is removed if no traffic is detected.

Example of a MAC Address Table

Process of Populating the MAC Address Table

Step-by-Step Learning Mechanism

1. Initial State: When a switch is powered on, its MAC table is empty.
2. First Frame Arrival:

• Device A (MAC: 00:1A:2B:3C:4D:5E) sends a frame to Device B.
• The switch records Device A’s MAC address and the incoming port (Fa0/1).

• If Device B’s MAC is not in the table, the switch floods the frame to all ports (except Fa0/1).

• Device B replies, and the switch now learns Device B’s MAC address and its port (Fa0/2).

• Future frames between Device A and B are forwarded only to the correct ports (no flooding).

Key Points for CCNA Exam

• The switch only learns from the source MAC address, not the destination.
• If a MAC address is not found, the switch floods the frame (broadcast/unknown unicast).
• Static MAC entries can be manually configured and do not age out.

Types of MAC Address Table Entries

Aging Time and MAC Address Table Maintenance

• Default Aging Time: 300 seconds (5 minutes).
• Why Aging? Prevents stale entries when devices move or disconnect.
• Viewing the MAC Table:

Show mac address-table

• Clearing Entries:

Clear mac address-table dynamic

Troubleshooting MAC Address Table Issues

Common Problems & Solutions

CCNA 2 Exam Focus: Key Concepts

• MAC learning process (source-based learning).
• Difference between dynamic, static, and sticky MAC entries.
• Aging time and its purpose.
• How switches handle unknown unicast/broadcast frames.
• Commands to view/manage the MAC table (show mac address-table).

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Conclusion

Understanding how a switch populates its MAC address table is fundamental for the CCNA 200-301 and CCNA 2 exams. The switch learns MAC addresses dynamically from incoming frames, updates its CAM table, and uses this information for efficient forwarding.

For the best exam preparation, Study4Pass provides high-quality study materials, practice tests, and expert guidance to help you pass your CCNA certification with confidence.

Final Tips for Success:

• Master the show mac address-table command.
  Know the difference between dynamic, static, and sticky MAC entries.
  Practice switch configurations in Packet Tracer or real labs.
  Use Study4Pass for verified exam questions and explanations.

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Sample Questions for Cisco 200-301 Exam Dumps

Actual exam questions from Cisco's 200-301 Exam.

1. What does a switch examine to learn and populate its MAC address table?

a) Destination IP address

b) Source MAC address

c) Destination port number

d) Source IP address

2. When a switch receives a frame, how does it update its MAC address table?

a) By recording the destination MAC address and incoming port

b) By recording the source MAC address and incoming port

c) By recording both source and destination MAC addresses

d) By flooding the frame to all ports

3. Which of the following does a switch NOT use to build its MAC address table?

a) Source MAC address of incoming frames

b) Port number where the frame arrives

c) Destination IP address of the packet

d) VLAN ID (if configured)

4. How does a switch handle an unknown destination MAC address?

a) Drops the frame

b) Floods the frame to all ports (except the incoming port)

c) Sends an ARP request

d) Forwards it to the default gateway

5. What happens to the MAC address table if a host moves to a different switch port?

a) The switch updates the MAC address table with the new port

b) The switch keeps the old entry and adds a new one

c) The switch deletes the old entry after a timeout period

d) The switch ignores the change until manually updated