Layer-3 switching technology for inter-VLAN Communication

Introduction

With the rapid development of information technology, especially the continuous improvement of computer and network technologies, people have higher and higher requirements for network transmission, in the competition, Ethernet stands out as the first choice for modern enterprise networks with its advantages in high transmission speed, low consumption, easy installation, and good compatibility, the emergence of VLAN technology solves the problem of network information overload. However, because the communication between different VLANs must rely on the routing function, the traditional router has its own low speed, complexity, and other limitations, it is easy to become the bottleneck of the network, making the advantages of Ethernet hard to exert. The emergence of layer-3 switching technology overcomes the disadvantages of traditional routing and solves this problem satisfactorily.

1. Necessity of layer-3 Exchange Technology Development

The main function of a traditional router is to achieve route selection and network interconnection, that is, to obtain the topology information of the subnet and the network characteristics of each physical line through a certain channel, and obtain the optimal path to each subnet through a certain routing algorithm. Create a route table to redirect each IP packet to the hop hoptohop. Second, it must process different link protocols. When an IP packet passes through each vro, it must go through software processes such as queuing, protocol processing, and addressing routing, resulting in increased latency. At the same time, the vro adopts the shared bus mode, and the total throughput is limited. When the number of users increases, the access rate of each user decreases. Vrouters focus more on multiple media types and transmission speeds. Currently, data buffering and conversion capabilities are more important than line rate throughput and low latency.

Compared with routing technology, switching technology has the advantage of fast speed. When the network size is large, high-speed and large-capacity routers are very necessary. On the other hand, because most modern communication networks use fiber technology, the main bottleneck of data networks is node routers. The current layer-3 switching, Routing Switching, and other terms are all examples of this idea. Although layer-3 switching was initially designed for LAN, it uses the destination IP address for switching, but now this technology has started to be used in wide area networks. It does not need to spread broadcast packets, but directly uses dynamic MAC addresses for communication, such as IP addresses and ARP, it provides routing functions based on IP and IPX protocols between multicast and virtual networks. The smooth implementation of this function mainly relies on the dedicated ic asic to change the commands processed by the traditional routing software to the embedded commands of the ASIC chip, thus accelerating the storage forwarding and filtering of packets, this ensures high-speed linear routing and service quality.

2. Basic principle and structure framework of layer-3 Exchange Technology

2.1 Basic Principles of layer-3 Exchange Technology

Layer-3 switching is a network technology that combines exchange and routing by introducing a routing module in a network switch instead of a traditional router. It flexibly segments the network on the second or third layer of the network based on actual application conditions. 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 a router device on a LAN switch.

The design of the layer-3 switch is based on the careful analysis of the IP route. It is a very simplified process to extract the processes that every packet in the IP route must pass through. Most of the packets in an IP route do not contain any option. Therefore, it is unnecessary to process the IP option in most cases. The length of packets in different networks is different. To adapt to different networks, IP addresses must implement the packet sharding function. However, in the Ethernet environment, the network frame length is fixed, therefore, packet fragmentation can be omitted. The layer-3 switching technology does not use the longest Address Mask matching method of the router, but uses the exact address matching method. In this way, it facilitates fast hardware search. It uses the high-speed cache method to store recently used host routes in the hardware lookup table. Only items that cannot match in the cache will be forwarded through the software. The stream exchange method is used in the storage and forwarding process. In the stream exchange process, the first packet is analyzed to determine whether it represents a stream or a group of packets with the same source address and destination address. If the first packet has the correct characteristics, the subsequent packets in the identified stream will have the same priority, and the subsequent packets in the same stream will be exchanged to the destination address based on the second layer, in order to achieve high-speed switching, the current layer-3 switches all adopt the stream switching mode. It improves the processing of IP routing, implements a simplified IP forwarding process, and uses a dedicated ASIC chip to implement hardware forwarding, in this way, the vast majority of message processing can be implemented in hardware. Only a few packets need to be forwarded by software. The forwarding performance of the entire system can be increased by thousands of times, the cost of devices with the same performance is also greatly reduced.

Each VLAN corresponds to an IP segment. L2 switches are isolated from each other, which is exactly the same as the switch engine in L2 switches. Access between different IP address ranges must span VLAN. The layer-3 forwarding engine must provide the inter-VLAN routing function. In a L2 Switch and vro network, each IP address segment that needs to communicate with other IP addresses must use a router interface as the gateway. The layer-3 forwarding engine is equivalent to a router in a traditional network. When you need to communicate with other VLANs, you must allocate a routing interface on the layer-3 switching engine for VLAN gateway. This routing interface on a layer-3 switch is implemented on the layer-3 forwarding engine and layer-2 forwarding engine by configuring a forwarding chip. Unlike the router interface, this interface is invisible. The following is an example to illustrate the communication process. Assume that two sites A and B that use the IP protocol communicate with each other through the layer-3 Switch. When sending A message, the sender A compares its IP address with the IP address of site B, determine whether Site B is in the same subnet as it is. If Site B and Site A are in the same subnet, layer-2 Forwarding is performed. If the two sites are not in the same subnet, if Station A needs to communicate with Station B, station A must send ARP packets to the layer-3 Switch module. When station A broadcasted an ARP request to the IP address of the layer-3 Switching Module, if the layer-3 Switching Module knew the MAC address of Station B during the previous communication, then, send A reply to the MAC address of Site B. Otherwise, the layer-3 Switching Module broadcasts an ARP request to Site B based on the routing information, after receiving the ARP request, site B replies its MAC address to the layer-3 Switch module. The layer-3 Switch module saves the address and replies it to site, at the same time, the MAC address of Site B is sent to the MAC address table of the layer-2 switching engine. After that, all data packets sent by A to B are forwarded to the layer-2 exchange process, enabling high-speed information exchange. It can be seen that layer-3 processing is only required in the routing process, and most data is forwarded through layer-2 switch. The speed of layer-3 switch is very fast and close to that of layer-2 switch.

2.2 simple topology structure of layer-3 Switching Technology

The devices used to connect to backbone switches include servers, switches, hubs, and workstations. The core switch is a layer-3 switch. It is used to divide two logical subnets with different functions to implement communication between different VLANs. As shown in figure 1, the layer-3 switch in the same VIAN virtual subnet only has the layer-2 switching function to ensure the transmission speed, the layer-3 Switch also acts as a layer-3 switch, which can correctly perform ARP resolution to ensure correct transmission of data streams. It also supports multicast, frame and packet filtering, traffic calculation, and other functions, to ensure security performance and user needs.

3. Advantages and practical value of layer-3 Exchange Technology

Exchange technology provides basic network services: Switching virtual circuits and permanent virtual circuits and other supplementary services, such as user groups and network user identification. During end-to-end communication between computers, supports routing selection and traffic control, and provides a variety of communication procedures, such as data forwarding, maintenance and operation fault diagnosis, billing, and network statistics. In addition to excellent performance, compared with traditional L2 switches, layer-3 switches with key equipment have better features, which can bring more advantages to lan and MAN network construction.

3.1 high scalability

When a layer-3 switch is connected to multiple subnets, The subnet only establishes a logical connection with the layer-3 Switch module. Instead of adding ports as traditional external routers do, it reserves interfaces for various extension modules, during network expansion, you can plug in modules to expand the network, which protects users' investment in devices such as local area networks and man networks and meets the needs of enterprise network expansion.

3.2 cost-effective

A layer-3 switch can connect to a large network and can basically replace some traditional routers. However, it is similar to a layer-2 switch because it is inferior to a traditional one.

3.3 supported protocols are flexible and compatible

In a LAN, layer-3 switches support the IP protocol and IPX protocol, which can basically meet the requirements. For routing protocols, you must carefully choose whether to support the routing protocol of small networks such as RIP, you must also consider whether to support routing protocols applicable to large and medium-sized networks such as 0SPF. At the same time, layer-3 switches also support the 802.1d protocol in large and medium-sized networks to ensure network robustness. 802.1d refers to the SpanningTree protocol. In large and medium-sized networks, redundant links are often used to ensure network connectivity, that is, to prevent network interruptions, there are multiple paths for a subnet to connect to the network trunk, but this will form a loop, so that data is always circulating in the network, thus blocking the network, and after using the Spanning Tree Protocol, the vswitch can detect and eliminate logical loops in the network, without compromising redundancy and ensuring network performance.

3.4 improve security

In the network, for the sake of security, data packets need to be filtered according to many rules to ensure that only data packets meeting the rules can pass through the layer-3 switch, because the communication and data transmission between different VLANs must go through the switch, the switch can adopt various security restrictions, and the current layer-3 switch supports the access control list, filters all data packets by line.

4. Application of layer-3 Exchange Technology

The Application of layer-3 switches is very simple. It is mainly used to replace traditional routers as the core of the network. Therefore, all areas that do not require WAN connection and require routers can be replaced by layer-3 switches. In Enterprise Networks and campus networks, layer-3 switches are generally used in the core layer of the network, and different subnets or VLANs are connected using the Gigabit ports or MB ports on layer-3 switches. Such a network structure is relatively simple, with fewer knots. In addition, more control functions are required and the cost is low. Its main applications include the following:

4.1 As the backbone switch of the Network

Layer-3 switches are generally used as backbone switches and server group switches of the network, and can also be used as network node switches. In the network, network administrators can work with other Ethernet switches to build a seamless 10/100/1000 (Mb/s) Ethernet Switching System, providing unified network services for the entire information system. Such a network system has a simple structure and features such as scalability and policy-based QoS.

4.2 PortTrunk technology supporting link Aggregation

In applications, there are often cases where Ethernet switches are connected to each other or when Ethernet switches are interconnected with servers. A single connection used for interconnection often becomes a network bottleneck. Using PortTrunk technology, you can logically think of several Ethernet connections of the same source switch and the destination switch as one connection line. This not only ensures that no loop occurs in the LAN, but also effectively increases the connection bandwidth. The layer-3 Switch with good performance fully supports PortTrunk technology, effectively meeting the requirements of enterprise LAN for connection bandwidth.

4.3 implement multicast and self-learning

In addition to the dynamic routing protocol RIP and OSPF, some third-level multicast protocols can also be implemented based on the standard multicast protocol to meet the increasingly popular multi-point multicast requirements, for example, Distance Vector multicast routing protocols such as DVMRP and PIM.

Conclusion

Although layer-3 vswitches have protocol dependencies. Most of them still need routers to complete some high-end routing functions. For example, gateways acting as VLANs to the WAN and other more complex routing requirements. Therefore, both the router and the layer-3 switch must maintain the route table, which obviously increases the burden of network management, and because China's communication infrastructure is relatively weak, traditional routing methods will also play a role for a long time in the future. However, as a new technology, it has a strong vitality and scalability. The third-layer exchange technology, from the concept proposal to today's application, has only been around for a few years, however, with the development of ASIC hardware chip technology, layer-3 switching technology and products will be further developed, it is also widely used in LAN, MAN, WAN, and other network exchanges.

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