Analysis of three-layer switching technology
Simply put, the Layer 3 switching technology is: Layer 2 switching technology + Layer 3 forwarding technology. It solves the situation that after the network segment is divided in the LAN, the subnet in the network segment must rely on the router for management, and solves the network bottleneck problem caused by the low speed and complexity of the traditional router.
What is three-layer switching Three-layer switching (also called multi-layer switching technology, or IP switching technology) is proposed relative to the traditional switching concept. As we all know, the traditional switching technology operates at the second layer of the OSI network standard model-the data link layer, while the three-layer switching technology implements high-speed data packet forwarding at the third layer in the network model. Simply put, the Layer 3 switching technology is: Layer 2 switching technology + Layer 3 forwarding technology.
The emergence of the three-layer switching technology solves the situation that after the network segmentation in the LAN is divided, the subnets in the network segment must rely on the router for management, and solves the network bottleneck problem caused by the low speed and complexity of the traditional router.
Layer 3 switching principle A device with a layer 3 switching function is a layer 2 switch with a layer 3 routing function, but it is an organic combination of the two and does not simply superimpose the hardware and software of the router device on the LAN switch.
The principle is: assuming that two sites A and B using the IP protocol communicate through a layer 3 switch, when the sending site A starts sending, it compares its own IP address with the IP address of the B station to determine whether the B station and its own Within the same subnet. If the destination station B and the sending station A are in the same subnet, Layer 2 forwarding is performed. If the two sites are not in the same subnet, if the sending station A wants to communicate with the destination station B, the sending station A sends an ARP (Address Resolution) packet to the "default gateway", and the IP address of the "default gateway" is actually Layer 3 switching module of layer 3 switch. When the sending station A broadcasts an ARP request to the IP address of the "default gateway", if the Layer 3 switching module has already known the MAC address of station B in the previous communication process, it replies to the sending station A with the MAC address. Otherwise, the layer 3 switching module broadcasts an ARP request to station B according to the routing information. After receiving this ARP request, station B replies to the layer 3 switching module with its MAC address. Layer 3 switching module saves this address and replies to sending station A, while sending The MAC address of the station is sent to the MAC address table of the Layer 2 switching engine. Since then, when the data packets sent from A to B are all handed over to the Layer 2 switching process, the information can be exchanged at high speed. Since only Layer 3 processing is required in the routing process, most of the data is forwarded through Layer 2 switching. Therefore, the speed of Layer 3 switches is very fast, close to the speed of Layer 2 switches, and it is much cheaper than the same router.
Types of Layer 3 switches Layer 3 switches can be divided into pure hardware and pure software according to the data they process.
(1) The three-layer technology of pure hardware is relatively complex in technology and high in cost, but it has fast speed, good performance and strong load carrying capacity. The principle is that the ASIC chip is used and the routing table is searched and refreshed in hardware. When the data is received by the port interface chip, it first searches for the corresponding destination MAC address in the layer 2 switching chip, and if it is found, it performs layer 2 forwarding, otherwise it sends the data to the layer 3 engine. In the three-layer engine, the ASIC chip looks up the corresponding routing table information, compares it with the destination IP address of the data, and then sends an ARP packet to the destination host to obtain the MAC address of the host and sends the MAC address to the layer two chip. The data packet is forwarded by the Layer 2 chip.
(2) The software-based three-layer switch technology is relatively simple, but the speed is slow, and it is not suitable for cooperation. The principle is to use the CPU to find the routing table in software.
When the data is received by the port interface chip, first look up the corresponding destination MAC address in the layer 2 switching chip, if it is found, it will perform layer 2 forwarding or send the data to the CPU. The CPU searches the corresponding routing table information and compares it with the destination IP address of the data, then sends an ARP packet to the destination host to obtain the MAC address of the host, sends the MAC address to the layer 2 chip, and the layer 2 chip forwards the packet . Because the processing speed of low-cost CPUs is slow, the processing speed of such three-layer switches is slow.
Market product selection In recent years, the construction of broadband IP networks has become a hot spot. The following uses layer 3 switch products suitable for positioning at the access layer or small and medium-sized aggregation layer as examples to introduce some specific technologies of layer 3 switches. The mainstream access layer 3 switches on the market include Cisco's Catalyst 2948GL3, Extreme's Summit 24, and Allied Telesyn's Rapier 24. These three layer switch products have their own characteristics and cover most of the application characteristics of layer 3 switches. Of course, when choosing a layer 3 switch, users can determine and select the above products or products of other manufacturers according to their own needs, such as the Nortel Network Passport / Acceler series, the original Cabletron SSR series (after Cabletron is divided into four, Most SSR Layer 3 switches have been incorporated into Riverstone), Avaya's CajunM series, 3Com's Superstack34005 series, etc. In addition, domestic network manufacturers Shenzhou Digital Network, TCL Network, Shanghai Broadcasting and Television should be sure that Ziguang Network Union, Capitel, etc. have all launched three-layer switch products. The following describes three of these products, so that you can more fully understand the three-layer switch, and choose the appropriate model for your situation.
The Cisco Catalyst 2948GL3 switch combines industry standard IOS to provide a complete solution. It fully supports IOS access control list ACL in version 12.0 (10) and above. With the core Catalyst6000, it can complete the construction of an end-to-end comprehensive broadband metropolitan area network. Its multi-layer switching service, and has stopped using RSM routing switching module, IOS version 6.1 and above fully supports ACL).
Extreme's three-layer switching product solution can provide a unique Ethernet bandwidth allocation capability. The cutting unit is 500kbps or 200kbps. Service providers can charge based on bandwidth usage and can achieve fixed delay transmission of audio and video.
The PPPoE feature provided by Allied Telesyn's Rapier24 layer 3 switch enriches and improves user authentication and billing methods. It can be suitable for a variety of access networks. It is flexible in application and easy to implement business choices. At the same time, it also protects existing users' existing investments. Functions such as NAT (Network Address Translation) and DHCP Server are optimistic for many service providers.
In short, the three-layer switch from the concept to today's popular application, although only after a few years, but its extended functions continue to be combined with practical applications to be enriched. With the development of ASIC hardware chip technology and the promotion of practical applications, the technology and products of the three-layer switching will be further developed.
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