Layer 3 switching and Routing

The rapid development of computer network technology and application has promoted the continuous improvement of the degree of social informatization, while the improvement of information demand has promoted the emergence of new network technologies. These technologies have greatly improved network scalability, flexibility, transparency, and speed. Among the numerous network products, switches play a vital role in building high-performance networks, and their technological development is also remarkable. Nowadays, the word "layer-3 switching" has become popular in the industry. In large and medium-sized networks, there are already many networks with layer-3 switches as the core. With the rapid development of China's enterprise network, campus network and broadband network, the third-layer switch has become a new market growth point, its applications also penetrate from the backbone layer and convergence layer at the initial network center to the access layer at the network edge. This field is no longer dominated by foreign vendors. Many domestic vendors, such as the TCL network and shenzhoudigital network, have launched their own layer-3 exchange products.
I. Development of Exchange Technology
The combination of computer technology and communication technology has promoted the rapid development of computer LAN. From the emergence of Aloha network in the late 1960s s to the debut of gigabit switched Ethernet in the late 1990s S, in just 30 years, it has experienced a leap from work order to duplex, from sharing to exchange, from low speed to high speed, from simple to complex, from expensive to popularization, from second-layer exchange to multi-layer exchange.
1. Layer 2 switching
When we first set up a LAN, it was mainly limited to host connection, file sharing, and print sharing. Multiple users can share 10 mbps bandwidth to meet these needs. As the network scale expands, the previous network system is no longer competent, because in the LAN, the earliest network interconnection device is a hub, which is the first physical layer) device. In this network based on the CSMA/CD physical layer protocol, user data conflicts often occur, resulting in re-transmission of data, greatly reducing the transmission efficiency. At that time, the second layer of data link layer was used as the device Bridge, which refined the network segment and reduced the conflicting domain, thus optimizing the LAN performance. However, the bridge is a device that is transparent to the upper layer (above Layer 3) protocol and cannot effectively prevent broadcast storms. Therefore, a router is required. Vrouters play a key role in subnet interconnection, security control, and broadcast storm restrictions. However, complicated algorithms and low data throughput make vrouters a network bottleneck. In order to solve the above problems, the industry has improved the Net Bridge and made a LAN switch, which can be used to replace the hub to improve the network performance.
A lan switch is a layer 2 network device that constantly collects and establishes its own MAC address table during operation and regularly refreshes the table. The introduction of this service enables dedicated bandwidth between sites on the network, eliminating unnecessary conflict detection and error re-transmission, improving transmission efficiency and transmitting user data point-to-point, other nodes are invisible. However, layer-2 exchange also has its weaknesses, including the inability to effectively solve problems such as broadcast storms, inter-network interconnection, and security control. Therefore, the VLAN Virtual LAN technology on the switch is generated.
2. layer-3 Switching
The L2 Switch works on the data link layer, the second layer of the OSI reference model. Its main functions include physical addressing, network topology, error verification, frame sequence, and traffic control. To improve the performance of a vswitch, a layer-3 vswitch is introduced. It adds many new functions on the premise of retaining all the functions of the layer-2 computer, such as VLAN support, link aggregation support, and even firewall features. To put it simply, the layer-3 Switch adds the routing function when dividing VLAN based on the protocol.
Layer-3 switches are the key to Intranet applications. They combine the advantages of layer-2 switches and layer-3 routers into a flexible solution, provides line rate performance at all levels. This integrated structure also introduces policy management attributes, which not only associates Layer 2 with Layer 3, but also provides traffic prioritization, secure access mechanisms, and other functions.
The layer-3 Switch consists of three parts: interface layer, switch layer, and route layer. The interface layer includes all the important LAN interfaces, such as 10/100 Mbps Ethernet, Gigabit Ethernet, FDDI, and ATM. The exchange layer integrates multiple LAN interfaces and is supplemented by policy management, it also provides link aggregation, VLAN, and tag mechanisms. The routing layer provides main LAN routing protocols, including IP, IPX, and AppleTalk, and implements policy management, provides the layer-3 forwarding technology for traditional routing or pass-through. The combination of policy management and administrative management enables network administrators to adjust networks according to specific enterprise needs.

Generally, layer-3 Switching Products Use programmable and scalable ASIC chip technology, which provides the following rich features:
(1) Non-blocking line rate switching and routing for all network interfaces and protocols on all ports;
(2) with a very high throughput, the packet forwarding speed is packet forwarding/per second, pps) is usually 10 ~ faster than the high-end router ~ 100 times;
(3) Multiple protocol routing options, such as ipr1_1/v2, OSPF), IP MulticastDVMRP, PIM), and IPX;
(4) supports the division of multiple VLANs Based on port/MAC address, protocol, IP subnet, IEEE 802.1Q, or Cisco ISL;
(5) traffic priority processing with reserved bandwidth RSVP) and service class CoS) and service quality QoS, supporting IEEE 802.1p and service classification DifferServ );
(6) You can set Access List Control filtering rules or firewall-based security policies;
(7) supports Point-to-Point Protocol (PPPoE) over Ethernet, supports Secure user authentication, and works with user billing to enhance user management features;
(8) support incremental distribution of Ethernet bandwidth units;
(9) ASIC's programmability, supporting technologies such as IPv6 and other future technologies to protect user investment.

Ii. Comparison between layer-3 switching and Routers
In the past, most of the data in the network was in compliance with the "80/20" rule, that is, only about 20% of the data packets in the network communicate with the central server or other parts of the enterprise network through the backbone router, 80% of network traffic is still concentrated in subnets of different departments. But now, the situation has undergone fundamental changes, and even formed the "20/80" rule. To cope with the increasing data traffic, shared media networks have been replaced by switching networks. This change has a direct impact on the traditional routers used for network segmentation. Because most of the data traffic spans the IP subnet, the router has actually become a bottleneck for network transmission.
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 use a certain routing algorithm to obtain the optimal path for each subnet, establish a route table, and then jump each IP packet to hop) to the destination. 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. Although the performance of a vro has also been improved recently, about 1 Mpps, the cost of using this vro is also astonishing.
Compared with the routing technology, the exchange 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 optical fiber technology, the main bottleneck of data networks is node routers. The current layer-3 Exchange, route exchange, or other terms are the results of this idea. Although layer-3 switching was initially designed for LAN, it uses destination IP addresses for switching, but now this technology has begun to be used in wide area networks.
Layer-3 switching plays an increasingly important role in the current network construction. It does not need to spread broadcast packets, but directly uses dynamic MAC addresses for communication, such as IP addresses and ARP, it has the routing function of multi-channel broadcast and virtual network based on IP and IPX protocols. The smooth implementation of this function mainly relies on dedicated ic asic ). Change the commands processed by traditional routing software to embedded commands on ASIC chip to accelerate packet forwarding and filtering, this ensures high-speed linear routing and service quality.
3. Application of layer-3 Switching
The Application of layer-3 switches is actually 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 layer-3 switches with Gigabit ports or MB ports. In this way, the network structure is relatively simple and the number of nodes is relatively small. In addition, it does not require many control functions and the cost is low.
In the current hot broadband network construction, layer-3 switches are generally placed in the center of the residential area and the Convergence layer of multiple residential areas. The emergence of layer-3 switches has shaken the status of enterprise routers. As routers govern the WAN, layer-3 switches will no longer dominate the LAN.
According to the current situation in China, layer-3 switches are gaining momentum. Fortunately, many domestic manufacturers have launched the third-tier Ethernet switch with good performance. Layer-3 switches have the following features in application.
1. Act as the backbone switch
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 a network that works with other Ethernet switches, network administrators can build a seamless 10/100/Mbps Ethernet Switching System to provide 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 service. Layer-3 switches provide QoS Services for networks, including priority management, bandwidth management, and VLAN switching. Policy-based QoS allows the network administrator to allocate bandwidth to different types of network traffic, including TCP/UDP sessions, by priority, without any loss of switching performance.

Due to application requirements, most of the backbone switches are Ge switches. Currently, most of the layer-3 switches are Ge switches. The 10/100 Mbps adaptive port and GE port can be provided to connect copper wires, you can also connect to the optical fiber and provide a high-performance backplane channel. The organic cabinet type of such switches can also be stacked and can be selected based on different situations.
2. Support for Trunk Protocol
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. The Trunk technology can be used to logically consider the Ethernet connection lines of several identical source switches and destination switches 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 the Trunk protocol, and some support eight groups of trunks, which can effectively solve the connection bandwidth problem in the enterprise LAN.

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 requirements for Multi-Point multicast, such as distance vector Multicast Routing Protocol DVMRP ).
The layer-3 Switch also supports self-learning. It can automatically discover the ing between the host's IP address and the connection port without using any routing protocol. Once a vswitch is connected to the network, it continuously learns the IP address and subnet mask information of all connected hosts by listening to ARP, RIP, and ICMP packets. Based on the learned information, the switch establishes and maintains the routing information in the route table and automatically provides routing services for all IP data packets.

4. Improve Security
In the network, for the transmitted data packets, for security reasons, data needs to be filtered according to many rules to ensure that only compliant data packets can pass through the layer-3 switch. Layer-3 switches support internal hardware filters to filter all data packets without compromising system performance, in addition, data packets can be filtered based on any content from Layer 2 to Layer 7.
The typical applications of layer-3 switches are shown in the figure on page 80th.
　
In the figure, it is a LAN with a three-level switch structure. The backbone switch is a layer-3 switch, which is used to divide four logical subnets with different functions ), the network management system is used to centrally control and manage the entire network, it includes monitoring, adjusting the running status of the network, automatically allocating the user's IP address, and counting the Internet information traffic and user usage. Devices that can be directly connected to backbone switches include routers, various servers, central workstations, and secondary switches. The second-level switch can be a common second-level switch. Level 3 switches can use Layer 2 switches or hubs.