1.13 Describe switching concepts

MAC Learning and Aging

In a network switch, MAC (Media Access Control) learning is the process by which a switch builds and updates its MAC address table. When a switch receives a frame, it examines the source MAC address of the frame and records which port it arrived on. This information is stored in the MAC address table (also known as the forwarding table or content-addressable memory, CAM table).

By learning the MAC addresses and the corresponding ports, the switch can forward frames more efficiently. Instead of broadcasting the frame to all ports, the switch uses the MAC address table to send the frame only to the specific port associated with the destination MAC address.

MAC aging is a process that helps the switch manage its MAC address table by removing entries that are no longer valid. Each MAC address entry in the table has an associated timer. If the switch does not receive any frames from that MAC address within a specified period (usually around 300 seconds), the entry is considered stale and is removed from the table. This prevents the table from becoming outdated and ensures that it only contains active and relevant entries.

Frame Switching

Frame switching refers to the process by which a network switch forwards frames from one port to another. When a switch receives a frame, it performs the following steps:

1. Examine the Destination MAC Address: The switch looks at the destination MAC address in the frame header.

2. Consult the MAC Address Table: It checks the MAC address table to determine which port the destination MAC address is associated with.

3. Forward the Frame: The switch forwards the frame out of the port that corresponds to the destination MAC address.

This method is efficient because it reduces unnecessary traffic on the network. Unlike hubs, which broadcast frames to all ports, switches direct traffic only to the port where the intended recipient resides.

Frame Flooding

Frame flooding occurs when a switch does not have an entry for the destination MAC address in its MAC address table. In this situation, the switch does not know which port to use to reach the destination. As a result, it floods the frame out of all ports, except the port on which the frame was received.

Frame flooding ensures that the frame reaches its intended destination, but it can lead to increased network traffic and congestion. Once the destination device responds, the switch will learn the correct port for the destination MAC address and update its MAC address table accordingly.

MAC Address Table

The MAC address table is a crucial component of a switch's operation. It stores mappings of MAC addresses to switch ports. This table allows the switch to efficiently direct traffic by providing a way to look up the appropriate port for any given MAC address.

The table is populated through MAC learning and updated as necessary through MAC aging and other mechanisms. Entries in the MAC address table have associated timers to ensure that they are removed when they are no longer valid. This dynamic updating helps maintain efficient switching operations and minimizes unnecessary traffic on the network.

Summary

Switching is a vital networking concept that enhances network efficiency and performance. Key aspects include:

• MAC Learning and Aging: Switches learn and store MAC addresses associated with specific ports, and periodically age out outdated entries to keep the table current.
• Frame Switching: Switches forward frames based on the MAC address table, directing traffic only to the necessary ports.
• Frame Flooding: When the switch doesn’t have a MAC address entry, it floods the frame to all ports to ensure it reaches its destination.
• MAC Address Table: This table maintains the relationship between MAC addresses and switch ports, enabling efficient frame forwarding.

By understanding these concepts, you can better manage and optimize network performance and troubleshoot switching-related issues effectively.