Layer 3 switching technology
1. introduction
In today's network construction, the newly emerged Layer 3 switch has become our first choice. It is recognized and praised by users for its efficient performance and excellent cost performance. At present, the three-layer switch has been widely used in many occasions such as enterprise network / campus network construction, intelligent community access, etc. The market demand and technology updates have promoted such applications to develop in depth.
2. Traditional switching technology
The traditional LAN switch is a layer 2 network device. It continuously collects information during operation to establish a MAC address table of its own. This table is quite simple and basically shows on which port a certain MAC address was discovered. In this way, when the switch receives an Ethernet packet, it will check the destination MAC address of the Ethernet packet and check its address table to confirm which port to send the packet from. However, when the switch receives a packet that it does not recognize, that is to say, if the destination MAC address is not in the MAC address table, the switch will "spread" the packet, that is, sent out from all ports, just as the switch receives a broadcast The same as the package, this exposes the weakness of the traditional LAN switch: it cannot effectively solve the problems of broadcasting, heterogeneous network interconnection, and security control. Therefore, the VLAN (Virtual Local Area Network) technology on the switch is produced.
3. Layer 3 switching technology
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 is operated at the second layer of the OSI network standard model-the data link layer, and the third layer of switching technology at the third layer of the network model achieves high-speed packet forwarding. Simply put, Layer 3 switching technology is "Layer 2 switching technology + Layer 3 forwarding". The emergence of the three-layer switching technology has solved the situation that the subnets in the network segment must rely on the router for management after the network segment is divided in the LAN, and solved the network bottleneck problem caused by the low speed and complexity of the traditional router.
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, rather than simply superimposing the hardware and software of the router device on the LAN switch. We can use the following examples to illustrate how Layer 3 switches work.
Suppose that two sites A and B that use the IP protocol communicate through a layer 3 switch. When sending site A starts sending, it will first compare its own IP address with that of station B to determine whether station B is in contact with itself. Within the same subnet. If the destination station B and the sending station A are in the same subnet, Layer 2 forwarding is performed. The specific steps are as follows: In order to obtain the MAC address of site B, site A first sends an ARP broadcast message to request the MAC address of site B. After the ARP request message enters the switch, it first learns the source MAC address. The chip automatically fills in the MAC address table of the site A's MAC address and the port number of the switch into the chip's MAC address table, and then performs the purpose in the MAC address table. Address lookup. Since this is a broadcast message, the switch broadcasts the broadcast message from the VLAN to which the switch port belongs. After receiving the ARP request message, site B will immediately send an ARP reply message. This message is a unicast message and the destination address is the MAC address of site A. After the packet enters the switch, similarly, the source MAC address is learned first, and then the destination address is searched. Since there is already a matching entry for the MAC address of site A in the MAC address table at this time, the switch directly transfers the packet from the corresponding port Forward it out. Through the last ARP process, the switch chip saves the information of sites A and B in its MAC address table. In the future, if A or B communicates or other sites on the same network segment want to communicate with A or B, the switch knows which port to send the packet from. It must also be noted that when searching the MAC address table, it is found that no matching entry is found, and the message is not a broadcast or multicast message. At this time, this message is called a DLF (DesTInaTIon Lookup Failure) message. The switch processes such packets as if it received a broadcast packet, and diffuses the packet from the VLAN to which the incoming port belongs. As can be seen from the above process, all Layer 2 forwarding is done by hardware. No software intervention is involved in the process of MAC address table learning or destination address lookup to determine the output port.
Let's take a look at how two sites implement cross-segment communication through Layer 3 switches.
As in the example above, Sites A and B communicate through Layer 3 switches. The network segments where Sites A and B belong are directly connected network segments on the switch. If Site A and Site B are not in the same subnet, the sending station A must first send an ARP request packet to its "default gateway". The IP address of the "provincial gateway" is actually the IP address of the VLAN to which site A belongs on the Layer 3 switch. When the sending station A broadcasts an ARP request to the IP address of the "default gateway", the switch returns an ARP reply message to the sending station A, telling the station A the MAC address of the switch, and at the same time, it can send the station A through the software. The IP address, MAC address, and port number directly connected to the switch are set in the three-layer hardware entry of the switch chip. After receiving the ARP reply message, Site A replaces the destination MAC address and sends the packet to B first to the switch. After receiving this packet, the switch will also first perform source MAC address learning and destination MAC address lookup. Since the destination MAC address is the MAC address of the switch at this time, the packet will be sent to Layer 3 of the switching chip in this case. Engine processing. In general, the three-tier engine will have two tables, one is the host routing table, this table is indexed by IP address, which stores the destination IP address, next hop MAC address, port number and other information. If a matching entry is found, the packet will be forwarded out the port specified in the table after performing some operations on the packet (such as the replacement of the destination MAC with the source MAC, TTL minus 1, etc.). If no matching entry is found in the host routing table, it will continue to search for another table-the network segment routing table. This table stores network segment address, next hop MAC address, port number and other information. Generally speaking, there are far fewer entries in this table, but the range of coverage is very large. As long as it is set properly, it can basically ensure that most packets entering the switch are forwarded by hardware, which not only greatly improves the forwarding speed, but also reduces CPU load. If no matching entry is found in the routing table of the network segment, the switching chip will send the packet to the CPU for processing and soft routing. Since site B belongs to one of the directly connected network segments of the switch, after receiving this IP packet, the CPU will directly check the ARP cache using the IP of B as the index. If there is no MAC address of site B, it will broadcast to station B according to the routing information. An ARP request. After receiving this ARP request, station B replies to the switch with its MAC address. At the same time that the CPU receives this ARP reply message, it can also use the software to change the IP address, MAC address, and port number of the switch into site B. The information is set to the three-layer hardware entry of the switching chip, and then the IP message sent from site A is forwarded to site B, thus completing the first one-way communication from site A to site B. Because the routing information of sites A and B has been saved in the three-layer engine inside the chip, in the future, the communication between sites A and B or the sites of other network segments want to communicate with A and B, the switching chip will directly take the package from The ports specified in the layer 3 hardware table entries are forwarded out without having to hand over the packets to the CPU for processing. This "one-time routing, multiple exchanges" approach greatly improves the forwarding speed. It should be noted that the routing table entries in the three-tier engine are mostly set by software. As for when and how to set, there is no fixed standard, we will not discuss in detail here. Generally speaking, a single-wave IP packet enters the Layer 3 switch and is forwarded through the following process:
Through the above process, we can understand the execution process of the message in the switch, and we can also clearly see how the three-layer switch fully combines the advantages of traditional switches and routers.
In the actual application network environment, the demand for cross-segment communication continues to increase. In the past, the network generally used the "80/20 distribution" rule, that is, only 20% of the traffic was through the backbone router and the central server or corporate network. The rest of the communication, and 80% of network traffic is still concentrated in different subnets. Today, this ratio has increased to 50% or even 80% (down to 28, 20/80). This is because today's network is experiencing the collective influence of many applications. Web applications have surpassed components and e-mail, and new applications have impacted the network so quickly and profoundly. For example, anyone can access a set webpage through any browser to support businesses such as sales, services, and finance. Functional data warehouse. This change has a direct impact on traditional routers. Because traditional routers pay more attention to supporting multiple media types and multiple transmission speeds, the current data buffering and conversion capabilities are more important than wire-speed throughput and low latency. The high cost and low performance of the router at the core of the network make it the bottleneck of the network, but it is indispensable because of the need for interconnection between the networks. Although high-speed routers have also been developed, they are only used for the Internet backbone due to their high cost. Layer 3 switches combine the advantages of layer 2 switches and layer 3 routers organically and intelligently to provide line-speed performance at all levels, thereby solving the network bottleneck problem caused by the low speed and complexity of traditional routers. Where there is no need for WAN connection, traditional routers used to connect different subnets are being replaced by Layer 3 switches at an extremely fast speed.
4. summary
Although the concept of three-layer switching has been popularized today, although it has only taken a few years, its application in network construction is becoming more and more extensive, from the initial backbone layer, the middle convergence layer to the edge of the connection Into the layer. Layer 3 switches have squeezed routers to the “edge†of the network due to their advantages of fast speed, good performance, and low price. Where there is no need for WAN connection and a router is required, a Layer 3 switch can be used instead. 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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