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Which information does a switch use to populate the MAC address table?

• the source MAC address and the incoming port
• the destination MAC address and the incoming port
• the source and destination MAC addresses and the incoming port
• the source and destination MAC addresses and the outgoing port
• the source MAC address and the outgoing port
• the destination MAC address and the outgoing port

The correct answer is the source MAC address and the incoming port.

Understanding the MAC Address Table in Switches

A MAC address table is a fundamental part of how a network switch operates. Switches, as Layer 2 devices in the OSI model, rely on MAC (Media Access Control) addresses to forward frames within a local area network (LAN). The MAC address table (also known as the Content Addressable Memory or CAM table) is a lookup table that switches use to make intelligent decisions about where to forward Ethernet frames.

Each entry in a MAC address table consists of:

1. A MAC address (the unique identifier of a device’s network interface)
2. The port of the switch through which the device is connected

Switches use the MAC address table to ensure frames are delivered only to the port where the destination device is connected, thereby increasing efficiency and reducing unnecessary network traffic.

The Role of the Source MAC Address and Incoming Port

When an Ethernet frame is received on a switch, the switch looks at the source MAC address in the frame’s header and associates that MAC address with the incoming port where the frame was received. This is how the switch learns which devices are connected to which ports. Over time, the switch builds up its MAC address table by learning the source MAC addresses from the frames it receives.

Here’s how the process works step by step:

1. Frame Reception: When a switch receives a frame on one of its ports, it immediately inspects the frame for its source MAC address.
2. Source MAC Learning: The switch uses the source MAC address and the incoming port to update its MAC address table. It records the MAC address and the port number where the frame arrived.
3. Table Population: If the source MAC address is not already in the MAC address table, the switch creates a new entry with the source MAC address and associates it with the port number. If the MAC address already exists but the port is different, the switch updates the port number to the new incoming port.
4. Forwarding Decision: Once the MAC address table is populated, the switch uses it to forward future frames. When a frame is destined for a specific MAC address, the switch consults its MAC address table to determine which port the destination MAC is associated with and sends the frame out only through that port.

Key Points About MAC Address Table Learning

1. Dynamic Learning: Switches dynamically learn and build their MAC address tables based on the source MAC addresses of the frames they receive. As devices transmit data, their MAC addresses are learned and added to the switch’s MAC address table.
2. Incoming Port Association: The key information the switch uses is the source MAC address and the incoming port. This allows the switch to map the MAC address of each device to the specific port where that device is located. For example, if a frame with the source MAC address 00:11:22:33:44:55 is received on port 2, the switch will add an entry in its MAC address table: 00:11:22:33:44:55 -> Port 2.
3. Unidirectional Learning: The MAC address learning process is unidirectional. The switch only learns from the source MAC address and incoming port, not from the destination MAC address. This is a crucial point because the destination MAC address is used for forwarding the frame, not for learning.
4. Aging: Entries in the MAC address table are typically not permanent. Most switches have an aging timer that will remove an entry if no frames are received from the corresponding MAC address for a certain period of time. This ensures that the table does not become outdated if devices are disconnected or moved to different ports.

Why the Switch Uses the Source MAC Address and Incoming Port

The reason why a switch uses the source MAC address and the incoming port is rooted in the way switches are designed to forward frames efficiently within a network. Let’s explore why these specific pieces of information are crucial:

1. Identifying the Device’s Location: The switch needs to learn which device is connected to which port in order to efficiently forward frames. The source MAC address provides a unique identifier for the device sending the frame, and the incoming port identifies which physical port on the switch that device is connected to.
2. Building the MAC Address Table: The MAC address table is essentially a mapping of MAC addresses to switch ports. By learning the source MAC address and associating it with the incoming port, the switch builds this mapping dynamically without the need for manual configuration.
3. Avoiding Broadcasts: If a switch didn’t have a MAC address table or it didn’t learn MAC addresses, it would need to flood every frame out all ports (except the one it came from), similar to how a hub operates. By learning the source MAC addresses and associating them with ports, the switch can intelligently forward frames to the correct port without broadcasting, thus minimizing unnecessary traffic.
4. Forwarding Decisions Based on Destination MAC Address: After learning where devices are located through the source MAC address, the switch uses the destination MAC address to make forwarding decisions. The switch consults its MAC address table to see if the destination MAC address is listed. If the destination MAC is found in the table, the switch forwards the frame out the port associated with that MAC address. If it’s not found, the switch floods the frame out all ports (except the one it came in on).

Common Scenarios in MAC Address Table Learning

To better understand how switches use the source MAC address and incoming port to populate the MAC address table, let’s look at a few real-world scenarios:

Scenario 1: A New Device Joins the Network

Suppose a new device (Device A) is plugged into port 1 of a switch, and it sends a frame to another device on the network (Device B). The switch receives the frame on port 1 and sees that the source MAC address of the frame is AA:BB:CC:DD:EE:FF. The switch now knows that Device A, with MAC address AA:BB:CC:DD:EE:FF, is connected to port 1, so it adds this information to its MAC address table.

Scenario 2: A Device Moves to a New Port

If Device A is unplugged from port 1 and connected to port 3, the switch will update its MAC address table accordingly. When the switch receives a frame from Device A on port 3, it sees that the source MAC address is AA:BB:CC:DD:EE:FF (the same MAC as before) but the incoming port has changed. The switch updates its MAC address table to reflect that Device A is now connected to port 3.

Scenario 3: Multiple Devices on a Single Port (with a Hub)

In some cases, multiple devices might be connected to a switch through a hub (an older networking device). In this case, multiple source MAC addresses could be learned on the same port. The switch will create separate MAC address table entries for each device’s MAC address but associate them all with the same port.

Importance of MAC Address Table Population

Accurate and efficient MAC address table population is vital for the following reasons:

1. Efficient Frame Forwarding: Without a populated MAC address table, the switch would need to flood all frames out all ports, causing unnecessary network traffic and inefficiency. By learning MAC addresses, the switch can forward frames directly to the correct port.
2. Minimizing Network Congestion: When switches know exactly which ports to send frames to, they reduce the amount of unnecessary data traveling across the network, preventing congestion.
3. Security: Switches can also use the MAC address table for security purposes, such as implementing port security features that limit the number of MAC addresses that can be learned on a particular port, helping to prevent MAC address spoofing attacks.

In conclusion, switches use the source MAC address and incoming port to populate the MAC address table, enabling them to forward frames efficiently and maintain an accurate understanding of the network’s topology.