Network administrators must not know the basic knowledge of L2 switches (1)

What we usually call lan exchange technology, also known as Layer 2 exchange technology, the content mainly includes the working principle of Layer 2 switching, network loops, and how to use the Spanning-Tree Protocol to solve Network loops, VLANs, and VTP technologies, this article mainly introduces the working principle of L2 switches and the dangers of network loops. Other content will be introduced later.

We all know that the traditional shared Ethernet uses the CSMA/CD mechanism, that is, the carrier listens for Multi-Channel Access/conflict detection. Let's analyze it in detail:

▲Csma/CD

CSMA/CD is a distributed media access control protocol. each node in the network can independently determine the transmission and receipt of data frames. Before sending data frames, each node must first perform carrier monitoring. frames can be sent only when the media is idle. At this time, if two or more nodes listen to the idle media and send frames at the same time, a conflict will occur. This will make all the sent frames invalid, and the sending will fail immediately. Each node must be able to detect conflicts at any time. In the event of a conflict, the transmission should be stopped to avoid wasted media bandwidth due to invalid frames transmitted. Then, after a random delay for a period of time, resend the media and resend the frame.

At first, three companies, Inter, Xerox and DEC, proposed the Ethernet standard. Later, the IEEE organization formulated the 802.3 standard, which stipulated the MAC Sub-layer of the Ethernet physical layer and the data link layer, it mainly defines 10BASE-2, 10BASE-5, 10BASE-T, 10BASE-F, etc., specifying the media, bandwidth, distance, and so on. At the same time, a 502.2 standard is defined to define the LLC sub-layer of the Ethernet data link layer, that is, the Logical Link Control Sub-layer, which mainly provides an interface between the data link layer and the network layer ,:

There are many protocols at the network layer. The data link layer provides a distinction between many protocols. Which protocol is used at the network layer? This is why the data frame encapsulates the header information of an LLC. However, in Traditional Ethernet 2 frames, instead of using LLC encapsulation, there is a two-byte type in the data frame to indicate the protocol used for Internet access. For example, the network layer protocol used is ARP.

Next let's take a look at the conflicting domain and broadcast domain.

Conflict domain: All nodes in a shared-bandwidth Ethernet can send data when they need to communicate with other nodes, but the CSMA/CD mechanism ensures that only one node can send data at a specific time, if the meat and meat send data at the same time, this will lead to a conflict, then such a range of conflicts may occur in this shared network, we are called a conflict domain.

Broadcast domain: A broadcast domain is a range that can be reached by a broadcast packet in a network. That is to say, if a broadcast packet is sent to a node on a network, the range that this broadcast packet can reach is called a broadcast domain,

As a professional network staff, the ability to identify conflicting domains and broadcast domains is an important skill. The first-layer device is added to expand the conflict domain. Only the second-layer device or third-layer device can be added to separate the conflict domain, that is to say, if a bridge, vswitch, and vro are added, the conflicting domain can be divided into smaller parts to reduce the bandwidth competition and conflict. A vro also has the advantage of dividing smaller broadcast domains. This is called network segmentation. Second-layer devices are used to reduce the range of conflicting domains, which is called "differential segments", that is, segments that are not meaningful. The true segmentation is that the CIDR block numbers are different and direct communication is impossible. Therefore, L2 devices can achieve segmentation, but the network cannot be truly segmented, while vrouters can achieve real network segmentation. Let's take a look at this sentence! A table is provided for your reference.

▲Network Segmentation

So our exchange technology is undoubtedly using a switch. Let's look at the function of a switch.