Application and development of L2/L3 Exchange

The application and development of L2/L3 Exchange is very active in the field of telecommunication and network technology. According to OSI layering, L2 corresponds to the data link layer, L3 corresponding network layer. In this paper, the second layer of Ethernet protocol and the corresponding third layer of IP protocol Exchange and routing technology are discussed.

Application and development of L2 Exchange

1. L2 Exchange principle and application

The concept of Exchange first appeared at the L2 level. Ethernet switches are equivalent to multiport high-density network bridges, all ports are in a L2 broadcast domain, and each independent port is a L2 conflict domain. Access to all hosts in the switch, at the beginning of communication with each other, will be broadcast mode (such as DHCP Discover broadcast packet), looking for the network segment of the DHCP server, automatic parameter configuration. At this time the switch through the Ethernet broadcast frame into the port of the source MAC address of the automatic learning function, to obtain the corresponding relationship between MAC address and switch port, so in the next step into the switch Ethernet frame, read the destination MAC address, find the corresponding port, Exchange. All the steps are done on the second level, the switch does not open the third layer, so it can be realized through hardware, and the exchange is fast.

Early Ethernet switching uses Half-duplex mode, and the CSMA/CD (Carrier Sense Multi access/collision Detect) Conflict detection mechanism must be enabled. In order to avoid two hosts simultaneously to listen to the network to know that there is no traffic and send data at the same time causing collisions in the Midway, the Ethernet transmission distance can be limited. The round-trip length (round-trip time) in a conflict domain must not exceed 512 bits of transmission time. Since the maximum RTT of the 0.01MS,FE network is 5.12ms for each bit of 100mbit/s Ethernet, it is possible to compute a CAT5 based UTP transmission distance of approximately 100m, and a 205 m maximum diameter local area network can be formed. Similarly, in the case of 10mbit/s Ethernet, the theoretical maximum diameter is 2050m without considering the electrical characteristics such as line decay. For FE Networks, a network can be achieved in one floor, several adjacent rooms, or several offices of the company. Therefore, the most L2 Exchange applications for local area network Exchange, as the name implies is in a small range. In this way, users usually use the PC host running the Windows network operating system, set up Peer-to-peer mode or domain mode Windows LAN, establish the file server, print server and email server, WEB server and database in the system. Server. Each host is typically configured automatically by DHCP.

The current Ethernet technology generally uses Full-duplex mode, no longer subject to the CSMA/CD mechanism, theoretically transmission distance can be infinitely far, such as the use of optical interface technology, the use of high-power optical interface and the low loss of optical fiber characteristics, the FE, GE interface transmission distance to a few kilometers or even dozens of kilometers.

In addition to LAN applications, many campus networks are also formed using L2 exchange networks. Because the L2 switch is hardware based bridging, in exchange, Ethernet frame forwarding is handled by ASIC chip, which can provide higher processing performance than traditional L2 Network bridge or L3 router, such as higher packet forwarding rate and lower forwarding delay. Therefore, L2 Ethernet exchange is widely used in ISP's pop node composition, also widely used in IDC network structure, providing high-speed data link between the Gigabit routers.

2. Limitations of L2 Exchange

Although the L2 exchange has many advantages, the L2 exchange is still not free from all the features and limitations of the bridge, and the broadcast domain established through the L2 switch still faces the same expansion and performance problems as the large bridging network in the past. In a specific application, the L2 broadcast domain has a 5-4-3 effect, that is, in a broadcast domain, allowing up to 5 L2 segment,4 Repeater/bridge, of which only 3 Segment can be loaded. As the number of hosts increases, the broadcast, multicast and flooding traffic in the network segment will increase, interfering with all the hosts in the network segment, consuming the CPU resources of the host, and consuming the LAN bandwidth for no reason.

In addition, L2 Exchange does not have L3 routing intelligence, can not achieve end-to-end multipath and load sharing, which is also the characteristics of the network Bridge decision. Therefore, in a L2 broadcast domain can not appear link loop, L2 switch need to enable the spanning tree protocol to eliminate L2 network formation loop, and in the main link failure, automatically provide backup path. At present, the link recovery time of spanning tree is not ideal, and it does not reach 50ms of telecom grade.

Secondly, in ISP network, if the L2 Exchange is used to provide the link between the core of the pop node and the convergence layer router, there is also a potential problem of decreasing the routing convergence performance. When an Ethernet switch fails between routers, it is possible that both the physical interface and the line protocol between the router and the switch are normal (Interface is up, lines protocol is up), routers are not in a position to perceive changes in network status, and require up to 40s of IGP PDU The Hello agreement is judged. For this reason, the current approach in ISP pop is to connect two routers directly to back-to-back connections through FE and GE cables.

3. The development of L2 Exchange

At present, the trend of L2 exchange is gradually moving from LAN range to metropolitan area and even wide area, especially in the most active city area. At present, starting from two domains infiltration to L2 Exchange technology and applications: SDH as the core of the city integrated transmission system, such as MSTP, supporting manufacturers have Alcatel, Lucent, Nortel, ZTE, Huawei and so on; a metropolitan Ethernet switch based on the L2/L3 switch, Support of the manufacturers have Cisco, Foundry, River Stone, Force 10.

The L2 Exchange products in two domains can expand the Ethernet broadcast domain from the area to the urban area, and provide the Ethernet transmission service in the metropolitan area. MSTP support points to multiple points of the poor, the device's L2 switching performance than the pure Ethernet switch. In addition, the MSTP provides Ethernet transmission through the encapsulation of the Ethernet frame, map to a number of VC-12 or VC-3, and then the VC-12, VC-3 cascaded and implemented, compared to the Ethernet switch network more flexible, more bandwidth utilization, you can use the optical mouth through bare fiber direct connection.

L2 Ethernet switching and transmission is provided in the metropolitan area, and the DSLAM network with Ethernet uplink interface is very adaptable. In the provision of ADSL Internet services, through the Metropolitan Ethernet Exchange or MSTP Ethernet transmission system, the decentralized set up at each end of the DSLAM through the long-distance Ethernet transmission channel converge to the relative set of bras. Reduce the cost of network construction, in line with the user's unified management, centralized authentication and billing strategy.

In the actual network, it is necessary to consider the performance, data flow, business type of MSTP or L2/L3 switch Group network comprehensively. At the same time, it is necessary to consider the factors such as network scalability, fault recovery and protection, VLAN support performance, network management capability, net Security and QoS. In general, although MSTP is slightly weaker in Ethernet switching and transmission performance, its ability to provide TDM circuits is currently L2 the network that the switch builds.