Switches are commonly used devices in network and desktop maintenance. Today, let’s discuss the working principles of switches:

Switches perform three types of forwarding operations: flooding, forwarding, and discarding.

1. Flooding: The switch forwards frames received on one interface to all other non-receiving interfaces. This is a point-to-multipoint operation (flooding occurs within a VLAN, and VLANs are isolated from each other).
2. Forwarding: The switch forwards frames received on one interface to another non-receiving interface. This is a point-to-point operation.
3. Discarding: The switch discards frames received on one interface without forwarding them.

Common Topics: Understanding Switches

Diagram Explanation

The blue arrows indicate the entry and exit of data packets, while the red arrows show the flow of data.

Generally, these three actions are collectively referred to as “forwarding” by switches. It’s important to consider the context to determine whether “forwarding” refers to a specific action or a general term.

MAC Address Table Concept

When a computer is connected to a switch port, if the computer starts sending data, the switch maps the computer’s network card MAC address to its interface. This mapping relationship is known as the MAC address table.

Viewing MAC Address Table on Huawei Switches

Command:

[Huawei]display mac-address verbose

This command helps identify which port a computer is connected to on the switch.

Scenario Example

Imagine a scenario where SW1 is connected to multiple non-managed switches (unconfigurable “dumb” switches), and you can only use PC2. You want to know which port on SW1 PC1 is connected to. The recommended method is to use the command to find the connected switch port. Using a cable tracer would require climbing and manually checking each port, which is more cumbersome.

Experiment

In the eNSP simulator, drag an S3700 switch without any configuration. Use auto mode for the cables, and the topology is as shown in the diagram.

Configure IP addresses:

• PC1: 10.1.1.1/24
• PC2: 10.1.1.2/24

You can see that PC2 is connected to the local port Eth0/0/2.

MAC Address Table Learning Process

When PC1 sends a data packet to PC2, the switch doesn’t initially know which port PC2 is connected to, only that PC1 is connected to e0/0/0. The switch binds PC1’s MAC address to e0/0/0 and duplicates the frame, sending it to all other non-receiving interfaces within the VLAN of the receiving port and the switch’s trunk interfaces. Each interface receives a copy. When PC2 receives the flooded frame, it replies to PC1, allowing the switch to learn the MAC address table.

The MAC address table can also be manually specified with the following command:

[Huawei]mac-address static AA-BB-CC Ethernet 0/0/22 vlan 1

Here, AA-BB-CC is the manually specified MAC address, followed by the port number and VLAN. Note that the VLAN must already exist on the device; it cannot be automatically created if it doesn’t exist locally, and an error will occur.

That concludes today’s knowledge sharing.