Comprehensive understanding of basic switch information

1. The concept of switch

Switch means "switch" and is a network device used for forwarding electrical (optical) signals. It can provide an exclusive electrical signal path for any two network nodes connected to the switch. The most common switch is an Ethernet switch. Other common ones include telephone voice switches, fiber optic switches, etc.

For example, a telecommunications broadband network cable is pulled into a university dormitory. There are 6 people in the dormitory. Everyone has a broadband account and wants to access the Internet. How can one network cable be turned into 6 network cables? At this time, you can use a switch to solve the problem. The switch provides a large number of ports for cable connection, so that star topology wiring can be used.

Interpretation: The network cable transmits electrical signals, and the optical fiber transmits optical signals. In addition to the RJ45 port, the switch also has an SC port that connects to the optical fiber. PCs connected to the switch can communicate with each other to form a LAN.

2. Working process

When each port is successfully connected, the switch forms a MAC table by matching the MAC address with the port. When receiving and sending data later, data packets sent to the MAC address will only be sent from the corresponding port.

Interpretation: After the terminal device is successfully connected to the switch port, the physical address/hardware address (Mac address) of the terminal device will form a corresponding relationship with the connected port and be recorded in the MAC table. The data packet contains Mac address information. When transmitting the data packet, the switch can send the data packet out from the corresponding port based on the address in the packet.

3. Functions of the switch

Learning: Correspond the Mac address of the connected device to the port and store it in the Mac table. The switch can "learn" the MAC address and store it in the internal address table. By establishing a temporary switching path between the originator of the data frame and the target recipient, the data frame can directly reach the destination address from the source address.

Forwarding/filtering: sending to the corresponding port according to the Mac address is sending, and not sending to other ports is filtering. When the destination address of a data frame is mapped in the MAC address table, it is forwarded to the port connected to the destination node instead of all ports (if the data frame is a broadcast/multicast frame, it is forwarded to all ports).

Eliminate loops: When a switch includes a redundant loop, the Ethernet switch uses the spanning tree protocol to avoid loops while allowing backup paths to exist.

Switch switching mode: The way a switch forwards data from one port to another is called switching mode.

4. Exchange mode type:

Store and Forward

Cut-Through

Fragment free

1) Store and Forward

Features: After receiving the data packet, the switch first stores the data packet into the buffer and performs CRC cyclic redundancy check. If the data packet has a CRC error, the packet will be discarded; if the data packet is complete, the switch queries the address. The mapping table forwards it to the corresponding port.

Advantages: There is no incomplete data packet forwarding, which can reduce potential unnecessary data forwarding. Disadvantages: The forwarding rate is slower than the direct forwarding method.

Applicable environment: Store-and-forward technology is suitable for network environments with ordinary link quality or poor quality. This method requires processing of data packets, so the delay is related to the size of the frame.

2) Cut-Through

Features: The switch only reads the first 6 bytes of the data frame, that is, searches the target address in the address mapping table, and transmits the data frame to the corresponding port. Cut-through switching can achieve less delay because the data frame is forwarded immediately after the destination address of the data frame is read and the forwarding port is determined.

Advantages: fast forwarding rate, reduced delay and improved overall throughput.

Disadvantage: It will bring many junk communication packets to the entire switching network.

Applicable environment: Network environments with good network link quality and fewer error packets. The delay time has nothing to do with the frame size.

3) Fragment discard (Fragmentfree)

Features: This is a solution between the first two. It checks whether the length of the data packet is enough to be 64 bytes. If it is less than 64 bytes, it means that it is a fake packet, and the packet is discarded; if it is greater than or equal to 64 bytes, the packet is sent.

Advantages: Data processing speed is faster than store-and-forward method.

Disadvantages: slower than pass-through.

Applicable environment: general communication links.