The third layer network Exchange technology realizes the inter-VLAN communication

With the rapid development of information technology, especially the continuous improvement of computer and network technology, people's transmission requirements for the network are more and more high, in the competition, Ethernet with its high transmission speed, low consumption, easy to install and good compatibility, such as the advantages of becoming the first choice for modern enterprise network, VLAN technology is a good solution to the problem of network information overload, but because the communication between different VLANs must rely on the routing function, and traditional routers due to its own low speed, complexity and other limitations, it is easy to become a network bottleneck so that the advantages of Ethernet is difficult to play, The emergence of the third layer switching technology overcomes the disadvantage of traditional routing and solves this problem satisfactorily.

1. The necessity of the development of the third-tier exchange technology

The main function of the traditional router is to realize routing and network interconnection, that is, to obtain the topology information of the subnet and the network characteristics of each physical line, and get the best path to each subnet through certain routing algorithms. The corresponding routing table is established so that each IP packet jumps (hoptohop) to the destination, and secondly, it must handle the different link protocols. When IP packets pass through each router, they need to be processed by the software such as queuing, protocol processing and addressing selection routing, which causes the delay to increase. At the same time, the router adopts shared bus mode, the total throughput is limited, and when the number of users increases, each user's access rate decreases. Routers are more focused on support for multiple media types and multiple transmission speeds, and the current data buffering and switching capabilities are more important than line-speed throughput and low latency.

Compared with routing technology, the advantage of switching technology is fast, when the network scale is very large, high-speed, high-capacity routers is very necessary. On the other hand, because the modern communication network mostly adopts optical fiber technology, the main bottleneck of the current data network is node router. Now the third layer of exchange, routing exchange or other related nouns are the embodiment of this idea. Although the third exchange was originally designed for the local area network, it uses the destination IP address to exchange, but now this technology has also begun to be used in the WAN. It does not need to spread the broadcast packet, but uses the dynamically established MAC address to communicate, such as IP address, ARP, and so on, it has the routing function between multicast and virtual network based on IP and IPX protocol. The successful realization of this function relies on the special integrated circuit ASIC to change the traditional routing software to the embedded instruction of ASIC chip, which accelerates the storage, forwarding and filtering of packets, and makes the linear Routing and service quality under high speed guaranteed.

2. The basic principle and structure framework of the third-tier switching technology

2.1 Fundamentals of the third-tier switching technology

The third layer Exchange is a network technology which introduces routing module in Network switch and replaces traditional router to realize the combination of exchange and routing. According to the actual application situation, it is flexible to network segmentation in the second or third layer of the network. The three-layer switching device is a second layer switch with a third-layer routing function, but it is an organic combination of the two, not simply to overlay the hardware and software of the router device on the LAN switch.

The design of the third layer switch is based on a careful analysis of IP routing, which is a very simple process to extract the process of each message in IP routing. The vast majority of messages in IP routing are messages that do not contain options, so in most cases the work of handling message IP options is redundant. The message length of different networks is different, in order to adapt to different networks, IP to achieve the function of packet fragmentation, but in the whole Ethernet environment, the network frame length is fixed, so packet fragmentation is also a work can be omitted. The third layer switching technology does not adopt the router's longest address mask matching method, but uses the accurate address matching method processing, thus is advantageous to the hardware realization quick Lookup. It uses the caching method to put the most frequently used host routes in the Hardware lookup table, and only items that cannot be matched in this cache will be forwarded through the software. In the process of storing and forwarding, a stream Exchange method is used, in which the first message is analyzed to determine whether it represents a stream or a group of messages with the same source address and destination address. If the first newspaper stationery has the correct characteristics, then the next message in the identification stream will have the same priority, the same class of subsequent messages are exchanged to the second layer based on the destination address, the current three-tier switch in order to achieve high-speed exchange, are used in the flow of switching mode. It has improved the processing of IP routing, the implementation of a simplified IP forwarding process, using a dedicated ASIC chip to implement hardware forwarding, so that most of the message processing can be achieved in the hardware, only a very small number of wisdom need to use software forwarding, the entire system forwarding performance can be increased thousands of thousand, Equipment of the same performance has also been significantly reduced in cost.

Each VLAN corresponds to an IP segment. On the two level, the VLAN is isolated, which is identical to the function of the switching engine in the two-tier switch. Access between different IP network segments spans VLANs to use the inter-VLAN routing capabilities provided by the three-tier forwarding engine. In a network of two-tier switches and routers, each IP segment that needs to communicate with other IP network segments needs to use a router interface as a gateway. Third-tier forwarding engine is equivalent to the traditional network routers, when the need to communicate with other VLANs, also in the three-tier switching engine to allocate a routing interface, used to do VLAN gateway. The routing interface on the three-tier switch is implemented on the three-tier forwarding engine and the two-tier forwarding engine by configuring a forwarding chip, which is not visible to the router's interface. Here is an example to illustrate the communication process. Let's say that both sites A and B using the IP protocol communicate through a third-tier switch, send station A at the beginning of the send, the IP address of their own and B station's IP address, to determine whether B station with their own in the same subnet, if the destination station B and send station A in the same subnet, then carry out two layers of forwarding, If two sites are not in the same subnet, such as sending station A to communicate with the destination station B, send station A to the three-tier switch three-tier switching module issued ARP (address resolution) packet. When send station a broadcasts an ARP request to the IP address of the three-tier switching module, if the three-tier switching module already knew the MAC address of B station in the previous communication process, reply to the MAC address of B of the sending station A, otherwise the three-layer switching module broadcasts an ARP request to station B according to the routing information, B Station received this ARP request to the three-tier switching module to reply to its MAC address, the three-tier Exchange module to save this address and reply to send station A, while the MAC address of B station sent to the two-tier switching engine MAC Address table. Since then, the packets sent by A to B are all handed over to the two-tier exchange process and the information is exchanged at high speed. Visible because only in the routing process requires three layers of processing, the vast majority of data through the two-tier exchange forwarding, three-tier switch speed, near the speed of two-tier switches.

2.2 Simple topological structure of the third-tier switching technology

The devices used to connect to the backbone switch are servers, switches, hubs, workstations, and so on. The core switch is a third layer switch, which divides two different functional logical subnets to realize communication between different VLANs. As you can see from Figure 1, the three-layer switch within the same Vian virtual subnet has only two-tier switching capabilities, in order to ensure the transmission speed of the requirements, and in different Vian subnets, the three-tier switch also plays a three-layer exchange function, can correctly ARP parsing to ensure the correct transmission of data flow, and it also supports multicast , frame and packet filtering, flow calculation and other functions to ensure security performance and user needs.