Ethernet switch purchasing skills and principles

There are many things worth learning about Ethernet switches. Here we mainly introduce the Ethernet switch purchasing techniques and principles. Currently, Ethernet switches use three packet exchange modes: pass-through, storage-forward, and fragment isolation when transmitting data packets from the source and destination ports.

1. pass-through switching

An Ethernet switch using a pass-through switching mode can be understood as a line matrix telephone switch with vertical and horizontal connections between ports. When a packet is detected on the input port, it checks the packet header, obtains the destination address of the packet, and starts the internal dynamic search table to convert it to the corresponding output port, connect the data packet to the corresponding port to implement the switching function. Because it only checks the packet header, usually only 14 bytes), does not need to be stored, so the cut-in method has the advantage of low Latency and fast switching speed. The so-called delay Latency) it refers to the time it takes for a packet to enter a network device and leave the device ).

It has three main disadvantages: first, because the data packet content is not saved by the Ethernet switch, it is impossible to check whether the transmitted data packet is incorrect and the error detection capability is not provided; second, because there is no cache, the input/output ports with different rates cannot be directly connected, and packet loss is easy. If you want to connect to a high-speed network, such as providing a Fast Ethernet 100BASE-T), FDDI, or ATM connection, you cannot simply "Connect" the input/output ports ", because there is a speed difference between the input and output ports, the cache must be provided. Third, when the port number of the Ethernet switch increases, the switching matrix becomes more complex and becomes more difficult to implement.

2. Storage and forwarding Methods

Storage forwarding Store and Forward) is one of the most widely used technologies in the computer network field. The controller of the Ethernet switch first caches the incoming data packets from the input port and first checks whether the data packets are correct, and filter out conflicting packet errors. After confirming that the package is correct, retrieve the destination address, find the output port address to be sent through the search table, and then send the package out. Because of this, the storage and forwarding method is insufficient because of the large data processing latency. However, it can detect errors of data packets entering the Ethernet switch, it also supports switching between input/output ports at different speeds, which can effectively improve network performance. Another advantage of this switching mode is that it supports switching between ports at different speeds to ensure collaboration between high-speed ports and low-speed ports. The solution is to store 10 Mbps low-speed packages and then forward them to the port at Mbps.

3. Fragment Free)

This is a solution between a pass-through and a storage and forwarding. Before forwarding, it checks whether the length of the data packet is 64 bytes (512 bits). If it is smaller than 64 bytes, it indicates a false packet or a residual frame), the packet is discarded; if the value is greater than 64 bytes, the packet is sent. This method processes data faster than the storage and forwarding method, but is slower than the pass-through method. However, it is widely used in low-end switches because it can avoid Frame Forwarding.

Switches using this type of switching technology generally use a special cache. This cache is a kind of FIFO ofirst In First Out), where bits enter from one end and then come Out from the other end In the same order. When a frame is received, it is saved in FIFO. If a frame ends with a length less than 512 bits, the content in the FIFO frame will be discarded. Therefore, there is no Frame Forwarding problem in a common direct-through forwarding switch, which is a very good solution. Data packets are cached and saved before forwarding to ensure that collision fragments are not transmitted over the network, which can greatly improve the network transmission efficiency.

Mainstream stack Exchange Technology

We have learned from the previous introduction that, if the switch works on the OSI/RM stack protocol layer, the current Ethernet switches mainly include Layer 2, Layer 3, and Layer 4 switches, they all have their own mainstream exchange technologies, which will be introduced below.

1. Layer 2 Exchange Technology

In early 1990s, a large number of LAN switches were introduced in the network system integration mode. A lan switch is a layer 2 network device. During the operation, the Ethernet switch constantly collects data to create its own address table. This table is quite simple, it mainly indicates the port on which a MAC address is found. When the switch receives a data packet, it checks the target MAC address of the packet and its own address table to determine the port from which the packet is sent. Instead of the hub, any sender data will appear on all the ports of the hub, whether or not you need it ). At this time, the switch can only work on the second layer of OSI/RM, so it is called the second layer switch. The technology used is also called the second layer switch technology ".

"Layer 2 switching" refers to the switch between the OSI Layer 2 or MAC layer. The introduction of Layer 2 switches enables dedicated bandwidth between network sites, eliminating unnecessary Collision Detection and error re-transmission, and improving transmission efficiency, in a vswitch, several independent and independent communication processes can be maintained in parallel. In the exchange network environment, user information is transmitted only between the source node and the target node, and other nodes are invisible. However, when a node sends a broadcast or multi-object broadcast on the Internet, or a node sends a MAC address packet that the switch does not know, all nodes on the Ethernet switch will receive this broadcast message. The entire exchange environment constitutes a large broadcast domain. That is to say, the second-layer switch may still have a "broadcast storm", which will greatly reduce the network efficiency, but the rate of the case is much less than that of the hub.

Layer-2 switching still has the weakness of "broadcast storm". At the same time, using layer-2 switching does not bring any progress to the functions of the router. As a result, Layer 2 switching can only improve performance in working groups that do not contain any vrouters locally. Between working groups that use layer-2 switching, the end-to-end performance of the router will be dropped due to router congestion, resulting in substantial performance degradation. Because of this, the third exchange technology of the routing method conforms to the needs of the times.

2. layer-3 Exchange Technology

In the technology of network system integration, the first-layer interface and second-layer exchange technology that are directly oriented to users have obtained satisfactory answers. However, as the core of the network, the router technology that acts as the interconnection between networks has no qualitative breakthrough. Traditional routers are software-based and have complex protocols. Compared with lan speed, their data transmission efficiency is low. At the same time, it serves as the hub for the interconnection of subnet and virtual networks, which makes the traditional router technology face severe challenges. With the rapid development of the Internet and Intranet and the wide application of B/S Browser/Server Computing Models, cross-region and cross-network services have increased dramatically, the industry and users are deeply aware of the bottleneck effect of traditional routers in the network, and it is imperative to improve the traditional routing technology. In this case, a new routing technology emerged, which is the third-layer switching technology. It is a router because it can operate on the third layer of the network protocol. It is a router that understands the device and can act as a route. It is a switch because it is extremely fast, almost reached the second-level switching speed.

A device with layer-3 switching is a layer-2 switch with layer-3 routing, but it is an organic combination of the two, it is not simply to overlay the hardware and software of the router device on the Ethernet switch. From the hardware implementation point of view, at present, the interface modules of the second layer switch exchange data through the High-Speed Backplane/bus speed up to dozens of Gbit/s. In layer-3 vswitches, layer-3 router-related Routing hardware modules are also inserted on the high-speed backplane/bus, this method enables the routing module to exchange data with other modules to be routed at a high speed, thus breaking the speed limit of the traditional external router interface by 10 Mbit/s-100 Mbit/s ). In terms of software, layer-3 switches also have significant initiatives, which define traditional software-based router software. Currently, layer-3 switches based on layer-3 switching technology have been widely used and have been unanimously approved by users.