Description of the forwarding process of the Layer 2 switch

This article explores the detailed process of how Layer 2 switches handle data forwarding within VLANs. By combining visual diagrams with textual explanations, we will examine how frames are structured and transmitted according to the 802.1Q VLAN protocol. Below is a diagram illustrating the specific forwarding mechanism of a Layer 2 switch:

Description of the forwarding process of the Layer 2 switch

The image above shows the step-by-step process of how a Layer 2 switch handles VLAN-tagged frames. Let’s now break down the key components of the 802.1Q frame structure. One of the most important fields in the 802.1Q tag is the **Tag Protocol Identifier (TPID)**. This 16-bit field has a fixed value of 0x8100, which identifies the frame as a VLAN-tagged frame. It serves a similar purpose to the Ethernet type field in untagged frames, helping the switch distinguish between regular and VLAN-tagged traffic. Next is the **Priority Code Point (PCP)**, a 3-bit field that defines the priority level of the frame, ranging from 0 (lowest) to 7 (highest). This is used for Quality of Service (QoS) management, allowing higher-priority traffic to be processed first. Another key field is the **Canonical Format Indicator (CFI)**, a single bit that indicates whether the MAC address is in a standard or non-standard format. In most Ethernet environments, this bit is set to 0, but it plays an important role in ensuring compatibility between different network technologies like Ethernet and Token Ring. Finally, the **VLAN Identifier (VID)** is a 12-bit field that specifies which VLAN the frame belongs to. The value 0 is reserved and typically means the frame does not belong to any VLAN, while values from 1 to 4094 can be used to identify up to 4094 unique VLANs. Now, let's take a closer look at how the switch table works in a VLAN-enabled environment.

Description of the forwarding process of the Layer 2 switch

In a VLAN-aware switch, the switch uses both the destination MAC address and the VLAN ID as the key for looking up the correct port to forward the frame. Like a standard switch, the VLAN switch can also learn MAC addresses dynamically through self-learning. To better understand VLAN operations, here are some important terms: - **PVID (Port VLAN ID):** When a port receives an untagged frame, it assigns the frame to a specific VLAN using the PVID. This essentially acts as the default VLAN for that port. - **TAGGED:** If a port is configured to send tagged frames, all outgoing frames will carry the VLAN tag, indicating they belong to a specific VLAN. - **UNTAGGED:** When a port is set to send untagged frames, the VLAN tag is removed before transmission. However, the PVID must be added to the frame before it is forwarded so that the receiving device can correctly interpret it. Understanding these concepts helps in configuring and managing VLANs effectively on Layer 2 switches.

2V Lead Acid Battery

First, basic concepts
AGM battery, the full name of Absorbent Glass Mat battery, that is, adsorbed glass fiber separator battery, is an innovative lead-acid battery. It uses a special glass fiber separator, which can effectively adsorb and fix the electrolyte, so as to achieve efficient and balanced charging and discharging of the battery.
​Second, technical characteristics
Efficient charging and discharging: Thanks to the special glass fiber separator, AGM batteries ensure that the electrolyte is evenly distributed inside the battery, thus improving the charging and discharging efficiency of the battery.
Sealed design: Most AGM batteries have a sealed design that eliminates the need for open exhaust, but instead regulates the internal and external air pressure through a pressure control valve. This design not only avoids the emission of acid mist, but also makes the battery more heat-resistant and reduces the performance degradation caused by excessive water loss.
Strong durability: The cycle charging capacity of AGM batteries is high, usually able to reach about 3 times that of lead-calcium batteries, so its service life is longer. At the same time, its capacitance also shows high stability during the whole service cycle.
Low temperature start reliable: The AGM battery in low temperature environment starting performance is more reliable, to ensure the normal use of the vehicle in cold weather.
Environmental protection: The design of the AGM battery reduces the emission of acid fog and pollution to the environment, which is in line with the modern environmental protection concept.
3. Application scenarios
Due to its superior performance characteristics, 2V AGM batteries are widely used in a variety of scenarios that require high-reliability and long-life batteries. For example, it can be used as a backup power supply to provide stable power support for key facilities such as communication base stations and data centers. It can also be used as a part of the energy storage system to store and release electrical energy to meet the power needs of different time periods.

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