Three-layer switch

Synonym core switch Generally refers to the three-layer switch

application background

For the convenience of security and management, mainly in order to reduce the hazard of the broadcast storm, large local logo must be used to function or region Waiting for a small local area network, which enables VLAN technology to be applied in a large number of applications, and communication between different VLANs must perform forwarding, with the increasing internet interviews. Simply use the router to achieve network access, not only due to the limited number of ports, but limits the size and access speed of the network. Based on this case, the three-layer switch is born, the three-layer switch is designed for IP, the interface type is simple, has a strong second-layer package processing power, which is very suitable for data routes and exchanges in large LAN, which can work The function of the traditional router is replaced or partially completed in the third layer of the protocol, and at the same time has a speed of almost second floors, and the price is relatively inexpensive.

In the enterprise network and teaching network, three-layer switches generally use the core layers of the network, connect different subnets or VLANs with a gigabit port or 100M port on the three-layer switch. However, it should be sober to recognize the most important purpose of the three-layer switch to speed up data exchange inside the large LAN, and the routing functions are also deployed around this purpose, so its routing features do not have the same grade professional router strong . After all, there are many lacks in terms of security, agreement support, and it is not fully replaced by routers.

During the actual application, the typical approach is to: in the interconnection of each subnet in the same local area, the route between VLANs in the local area network, with a three-layer switch instead of the router, and only the local area network and public The network interconnection is to achieve a professional router when enabled across local network access.

Operation mechanism

In the LAN, the three-layer switch follows the following operation mechanism: Change the legacy directive of routing software, add ASIC unique embedded chip and here Set the command. Check the existing routing table by means of hardware, with refresh features. In the process of data delivery, the port chip receives and identifies a piece of information, and the second layer chip analyzes the corresponding unique address. If you find a clear address, you will be forwarded again; if it is not possible to find it, the information is assigned to a subsequent engine.

The information associated with the routing table can be inspected, which matches the initial IP; the packet is sent to the host in the device to acquire a precise MAC. After these processes, the MAC is again transmitted again, and the second layer chip is forwarded. It can be seen that the three-layer switch has quickly accelerated the exchange rate of the normal case, from the rate to see it very approximately the two-layer architecture, and reduces the expenditure of the purchase router.

Comparison, the Layer 2 switch transmits broadcasts by means of a packet, but the three-layer switch can identify additional information, such as ARP and an address. In this state, the three-layer switch parses the broadcast packets of each partition to meet the quick requirements and do not have to truly spread the preset broadcast package. This design path can be subdivided to existing virtual subnets, depending on the three-layer route to deliver information about each other, communication will be more convenient.

Applicable purpose

To say that the role of three-layer switches in many network devices is not over. In the campus network, the urban education network, from the backbone network, the urban network backbone, the gathering layer has three floors, especially the core backbone network must use a three-layer switch, otherwise thousands of networks The computer is in a subnet, not only without security, but also cannot isolate the broadcast storm because of the unable to split the broadcast domain.

If a traditional router is used, although the broadcast can be isolated, the performance is not guaranteed. The three-layer switches have very high performance, and both three-layer routing function, which has two-layer network speed. Layer 2 exchange is based on MAC addressing, three-layer exchange is forwarding traffic based on the third layer address; except for the necessary routing process, most of the data forwarding processes are swapped by Layer 2, improve the efficiency of packet forwarding. .

The three-layer switch implements the routing function of IP by using the hardware switching mechanism. Its optimized routing software improves the efficiency of the routing process, solves the speed problem of the traditional router software routing. Therefore, it can be said that the three-layer switch has a "function of the router, the performance of the switch".

Connecting subnet

The computer on the same network is more than a certain number (usually in 200 sets, depending on the communication protocol), it is likely to be a large number of broadcasts on the network. The network transmission efficiency is low. In order to avoid broadcasting storms caused by broadcast on large switches, it can be further divided into multiple virtual networks (VLANs). But doing so will result in a problem: communication between VLANs must be implemented through a router. However, the traditional router is also difficult to compete between communication tasks between VLANs, because the network traffic relative to the local area network is too weak, and the routing capacity of traditional ordinary routers is too weak.

and the price of the Gigabit router is also very difficult to accept. If you use a gigabit port or a 100M port on a three-layer switch to connect to different subnets or VLANs, under the premise of maintaining performance, there is a problem that the subnet must rely on the router to communicate between the subnets after the subnet division. Therefore, the three-layer switch is an ideal device that connects the subnet.


Three-layer switch has powerful routing transmission, bandwidth allocation, multimedia transmission and security control functions, can divide different user groups according to different communication service systems, implement electricity Efficient transmission of industry business is important.

In addition to excellent performance, the three-layer switch also has some of the traditional two-layer switches, which can bring many benefits to the construction of the campus network and the urban education network, listed below.


When connecting multiple subnets, the subnet is only established with the third floors, and does not add ports like traditional external routers. It has thus protects the user's investment in the campus network and the metro education network. And meet the needs of rapid growth of network applications from 3 to 5 years.

High cost-effective

The three-layer switch has the ability to connect large networks, which can basically replace certain traditional routers, but the price is close to the Layer 2 switch. A 100m-layer switch price is only tens of thousands, and the price of the high-end two-layer switch is similar.

Built-in security mechanism

The three-layer switch can achieve one-way or bidirectional communication between different VLANs as the normal router, with the function of accessing the list. If set in the access list, you can limit the user to the specific IP address so that the school can prohibit students from accessing unhealthy sites.

Access list can not only be used to ban internal users from accessing certain sites, or to prevent illegal users from outside the campus network, the exterior of the Urban Areas Education Network access to the network resources inside the campus network, the interior of the metropolitan education network, Thereby improving the security of the network.

Multimedia Transport

Education Network often needs to transmit multimedia information, this is a characteristic of education network. The three-layer switch has the control function of QoS (service quality), which can assign different bandwidth to different applications.

For example, when the video flow is transmitted in the campus network, during the Metro Education Network, you can specifically reserve a certain amount of dedicated bandwidth for video transmission, which is equivalent to opening a dedicated channel in the network, other applications The program cannot occupy these reserved bandwidth, so it is possible to ensure the stability of the video stream transmission. The ordinary Layer 2 switch does not have this characteristic, so when transmitting video data, there will be a video and suddenly slowed jitter phenomenon.

In addition, video on demand (VOD) is also often used in education networks. However, since some video-on-demand systems use broadcasts to transmit, the broadcast package cannot achieve cross-network segments, so that the VOD cannot achieve cross-network segments; if the VOD is implemented in the form of unicast, although cross-network segment, but support At the same time, there is very little connection, and dozens of connections take up all bandwidth. The three-layer switch has a multicast function, and the VOD packet is transmitted in various subnets in the form of multicast, which enables both network segment transmission, and the performance of the VOD is guaranteed.

Billing function

In college campus network and the urban education network of some regions, it is likely to have a billing demand, because the three-layer switch can identify IP in the packet. Address information, therefore can statistically data traffic in the network, which can be charged according to traffic, and the computer can also be counted on the network, and billing is performed. And ordinary Layer 2 switches are difficult to do this.

Development Trend

In recent years, the three-layer switch continues to expand the original market, which is adopted quickly. This new product is popular, and the new switch is widely located in the local area network, smart community, metropolitan area network in a certain section. The network covers a wider range, and the current demand also shows the overexpressed overall trend. Under the driving of new technologies, the network highlights the situation of depth extension. Existing applications mainly include: network remote teaching, online shopping and watch video, community theater. The network has been continued, and the distributed routing is added to the bobbone of the distribution layer and the coincidation. Relying on the novel mode, the distribution route reduces the traffic of the edge, which is convenient for all preset network policies. From essentially, the inner architecture of the three-layer switch is simpler, which replaces the routing device in the conventional architecture. It can be seen that this new switch can be arranged if the WAN is not required to set up a wide area network but there is a routing function.

The switch is indispensable in the local area network. From the architecture of the arrangement, the three-layer switch is equipped with a new port such as a Gigabit, and is connected to the VLAN. The trend is: Broadband network construction: The category is wider, and the center center has integrated such a switch. Analysis of the perspective of the layered design unique, the three-layer switch can be promoted and adopted.

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