Switches are the unsung heroes of network connectivity. Their primary function is to connect devices to the network, but they have a multitude of other features to support this. When we think of switches, we think of layer 2. Layer 2 is all about MAC addresses, Ethernet frames, and VLANs.
In modern networks, switches also support some layer 3 functions, like IP addressing and routing. However, for this section, let’s focus on layer 2.
Ethernet protocols are commonly used in our networks, and each Ethernet frame has a source MAC address and a destination MAC address. Layer 2 is responsible for delivering frames from one hop to the next. With each hop, the source and destination MAC addresses are updated. This process is known as frame rewrite.
It’s important to note that switches don’t count as hops in the traditional sense. They don’t update the source and destination MAC addresses of each frame that passes through them.
So, what are switches for? They are the connectivity solution for all devices in a network. Instead of physically connecting every device to every other device, we connect devices to switches. This simplifies the network infrastructure significantly.
When a frame arrives at a switch, the switch determines which link or interface to send the frame out of. This ensures that the frame reaches its correct destination. But how does the switch know which link to use?
The switch accomplishes this by learning the MAC addresses of connected devices and the interfaces they are connected to. These details are stored in a MAC address table. When a frame arrives at a switch port, the switch looks at the destination MAC address in the Ethernet header and checks it against the entries in the MAC address table. Based on the table, the switch forwards the frame out of the appropriate port.
But what happens if the MAC address is not in the MAC table? In that case, the switch floods the frame out of all ports, ensuring that the frame is received by the intended device. Most devices will ignore the frame if it’s not meant for them. However, the correct device will respond, allowing the switch to learn which interface it is connected to and update the MAC address table accordingly.
The MAC address table is a vital tool for a switch. From the command line interface (CLI), you can use the command “show mac address table” to view all the MAC addresses known to the switch and the corresponding ports for reaching them. However, entries in the table are not permanent. A timer starts when an entry is learned, and if more traffic involving that MAC address is seen, the timer resets. But if the timer expires, the entry is removed from the table. This ensures that the table remains fresh and up-to-date.
Before we wrap up, if you’re preparing for the CCNA exam, we have some extra resources to help you study. These resources include study notes, quizzes, flashcards, transcripts, and command line summaries. You can find the link to these resources in the video description.
Feel free to check out the quizzes in this video, as they are available to everyone. They will give you a taste of what to expect in your CCNA journey.
So, next time you see a switch, remember its crucial role in network connectivity. It’s the unsung hero that connects devices, supports layer 2 operations, and ensures frames reach their intended destinations.
