Basic Principles of LAN switching

There are many things worth learning about lan exchange technology. Here we mainly introduce the basic principles of LAN exchange technology, as well as the knowledge of exchanging data frames and constructing and maintaining exchange address tables. LAN switching technology is a technology on the Data-Link Layer of the Data Link Layer in the OSI reference model. The so-called "switching" actually refers to forwarding Data frame ). In data communication, all switching devices (switches) perform two basic operations:

Exchange data frames, forward the data frames received from the input media to the corresponding output media, maintain the exchange operation, and construct and maintain the exchange address table. Next, we will discuss the details of these two basic operations.

LAN switching technology: exchanging data frames

The switch forwards data frames based on the MACMedia Access Control Address (physical address) of the data frame. When a vswitch forwards data frames, it follows the following rules: If the destination MAC address of the data frame is a broadcast address or multicast address, it forwards the ports except the data frames to all ports of the vswitch ); if the destination address of the data frame is a unicast address, but this address is not in the Address Table of the switch, it will also forward all ports except the data frame ); if the destination address of the data frame is in the Address Table of the switch, it is forwarded to the corresponding port according to the address table. If the destination address of the data frame and the source address of the data frame are in a network segment, it will discard the data frame, and the exchange will not happen.

Lan exchange technology: constructing and maintaining the address exchange table

In the switch address table, a table item consists of a host MAC address and the switch port number in which the address is located. The generation of the whole address table adopts the dynamic self-learning method, that is, when the switch receives a data frame, it records the source address and input port of the data frame in the exchange address table. InCisco switchesThe switch address table is placed in the Content Addressable Storage Content-Addressable Memory, CAM). Therefore, it is also called a CAM table.

Of course, before storing the exchange address table items, the switch should first check whether the source address matches the table items in the address table. Only when the match Table item does not exist can the table items be stored. Each address table item has a time mark to indicate the time period of the table item storage. Each time an address table item is used or searched, the time mark of the table item is updated. If the address table item is still not referenced within a certain period of time, it will be removed from the address table. Therefore, the address exchange table maintains the most effective and accurate address-port information.