Selection of broadband network switches

Recently opened the route for a new internet café, the Internet Café administrators ran to ask me, Internet cafes in the computer interconnection of the selected cable (hub) Good or switch good, Cisco's switch is not the best. People often say that only the right choice is not expensive. But how is the right choice? The common medium and small-sized network has two hundred or three hundred computers, many people reach thousands of units. They are connected by integrated cabling system switching devices. and connected to a broadband network via a switch or router. Therefore, the selection and use of various switches will have a very important impact on the performance of the broadband network system. I discuss and communicate some of the switch issues that are common in my work.





the difference between a hub and a switch





1. Hubs (this refers only to non-switched single and multi-segment types) are the first layer of the OSI architecture in the OSI architectures, the switch, which belongs to OSI's second tier of data link layer devices, is now a common three-tier exchange that provides VLAN and ip-based routing and switching on a two-tier platform, The four-tier switch is a port based application. A hub is just a synchronization of data transfer, amplification and reshaping of the role of the data transmission of short frames, fragments, etc. can not be effectively processed, can not guarantee the integrity and correctness of data transmission, similar to a large total linear lan; The switch can not only transfer the data to synchronize, enlarge and reshape, and can filter short frames, fragments to package packet forwarding and so on.





2. In terms of the way we work, a hub is a broadcast mode, that is, when a port of a hub is working, all other ports are able to listen to information, easily generate a broadcast storm, and each time only one port sends data, in addition to security is poor, all network cards can receive the data sent, Only the non-destination NIC discarded the envelope. When the switch is working, only the requesting port and destination port are responding to each other without affecting the other ports, so the switch can isolate the conflict domain and effectively inhibit the generation of broadcast storms.





3. In terms of bandwidth, a hub, regardless of the number of ports, all ports share a single bandwidth, at the same time only two ports to transfer data, other ports can only wait, while the hub can only work in Half-duplex mode, and for the switch, each port has an exclusive bandwidth, When two ports work without affecting the work of other ports, the switch can work in Half-duplex mode and can work in Full-duplex mode.





4. The switch works at the data link layer to address the MAC addresses, there is a certain additional addressing overhead, in the data traffic hours, the delay may be relative to the data transmission time is larger; The hub works at the physical layer to transmit data for broadcast mode, and the traffic hour performance drop is not obviously suitable for sharing the total linear structure LAN.




The difference between the
two or two-layer exchange and the third-tier exchange and the router





second-tier switching technology works on the data link layer. It forwards the corresponding port in the internal address table according to the destination MAC address of the packet received, and records the MAC address of the packet with the corresponding port in the internal Address table, and the MAC address is not in the table and waits for a response. Thus, the two-layer switch has the learning function for the MAC address, is transparent to the network layer or the high-level Protocol, the data exchange relies on the ASIC of the special Processing packet forwarding (application special integrated chipset) to realize the speed quickly. However, it cannot deal with three-layer and three-layer protocols and can not handle data exchange between different IP subnets.


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third-tier exchange works in the third layer of the OSI seven-layer model, is to use the IP packet header information in the three layer protocol to mark the following data stream, to reorganize the frame head, to exchange the message with the same mark data stream to the data link layer, that is to provide a data channel between the target address and the source address. Therefore, the three-layer switch can judge the routing without unpacking, thus forwarding the packet directly and exchanging data. Thus, different subnet IP packet Exchange can be realized. The other three-tier routing module is not a simple stack of two-layer switches and routers, it is a three-layer routing module overlay Two-layer switching high-speed backplane bus rate up to gbit/s, most of the required routing software processing data forwarding for three-tier forwarding, the rest are two-tier high-speed forwarding.


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The
router works in the OSI third layer network layer, and the working mode is similar to two layers. Routers primarily determine the best route and forward packets. There is a routing table within the router that records various link information for routing algorithms to compute the best route to the destination. The router then forwards the data. If the destination route is not known, the package is discarded and information is returned to the source address. Routers can learn routing information from each other or broadcast their own link state, so that routing information is updated in a certain way, thus the best route is computed by the algorithm. Therefore, the router path calculation is a lot of work. Routers have a limited number of general ports and slow routing forwarding. In the network data flow is large, and require fast forwarding response, it is often recommended to use the three-tier switch, and the Internet routing work to the router to complete.





III. switch Classification and selection index





from the transmission media and transmission speed, LAN switches can be divided into Ethernet switches, Fast Ethernet switches, Gigabit Ethernet switches, FDDI switches, ATM switches and Token ring switches, and so on, these switches are suitable for Ethernet, Fast Ethernet, FDDI,　 An environment such as ATM and Token Ring networks.





according to the broadest common classification method, LAN switches can be divided into desktop switch (Desktop switch), group switch (Workgroup switch) and campus network switch (campus switch) three categories.





according to the architecture, people also divide LAN switch into rack type, with expansion slot fixed configuration type, without expansion slot fixed configuration Type 3 kinds of products.





Select the broadband switch, in addition to the above introduction should also refer to the following key indicators:





1. Forwarding Technology





forwarding is the forwarding mechanism used by the switch to determine how packets are forwarded.





Express Forwarding Technology




When the
switch obtains the packet destination address, it begins sending packets to the destination port. Typically, the switch is aware of the destination address when it receives the first 6 bytes of the packet, allowing it to decide which port to forward the packet to. Pass-through forwarding technology has the fast speed, little delay and high throughput rate. However, when the error rate is high in the network, the switch forwards all the full packets and error packets, which brings many error packets to the entire switched network. The direct forwarding technology is suitable for network environment with good network link quality.


Storage and forwarding technology





Storage and forwarding technology requires the switch to decide how to forward after receiving all packets. This allows the switch to check packet integrity and correctness before forwarding. The advantage of this is that there is no incomplete packet forwarding, which reduces the potential for unnecessary data forwarding. Its disadvantage is that the forwarding rate is slower than the direct forwarding technique. Therefore, the storage and forwarding technology is more suitable for the common link quality network environment.





2. Back panel throughput and buffer size





Back-plate throughput is also called Backplane bandwidth, the unit is the number of packets per second (PPS), representing the switch interface processor or interface card and the data bus can huff and puff the maximum volume. The higher the bandwidth of a switch's backplane, the stronger the ability to process data. The maximum theoretical value is the line speed, which means that the switch can handle the packet forwarding buffer size of various sizes at full speed, also called the packet buffer size, and is a data queuing mechanism used by the switch to match the speed between different network devices. Data sent by a device with a high rate can be stored in a buffer until it is processed by a slow device. Buffer size by the buffer scheduling algorithm, the excessive buffer space requires a relatively large number of addressing time, buffer space is too small in the event of congestion will cause loss of the packet error.





3. Delay





Switch Latency is the time interval between the receipt of packets from the switch and the start of replicating packets to the destination port. There are many factors that affect the latency size, such as forwarding technology, buffer size, and so on. There is a fixed delay for the switch using pass-through forwarding technology. The switch with store-and-forward technology is related to packet size because it has to receive the full packet before it can start forwarding packets. The delay has a serious effect on the real-time and non-real-time video and voice information in the triple-net integration, which will cause the different steps of the picture and the voice.





4. Management features





for the convenience of network administrator management, and user control access to the switch, usually the switch should support SNMP MIB I/MIB II Statistical management function to meet common network management software such as OpenView, SUN solstice Domain Manager or IBM Network Management (NETVIEW) remote management switch. Complex switches also increase the ability to support RMON active monitoring through the built-in RMON group (Mini-rmon). or provide remote monitoring of the device through Web pages, command-line (CLI), to eventually implement common management functions such as fault management, performance management, configuration management, and security management.





5. MAC Address Table size and MAC address type





Each port or device connected to the LAN requires a MAC address that other devices use to locate specific ports and update routing tables and data structures. The MAC Address Table size reflects the ability of the device to support the number of nodes. A single MAC address type switch connects the end user or the non bridging device, and can not connect to the network segment of multiple networks such as hubs. Multi-MAC address switches can save multiple MAC addresses on each port with strong multi-node support capability.





6. Extended Tree





provides a redundant backup link for critical data links in order to secure the security of the network, because the switch is actually a transparent bridging device for multiple ports, thus triggering a "topological ring" problem. The switch automatically prevents the topology loop phenomenon by using the Extended Tree protocol algorithm to make each bridge device in the network aware of each other. Switch and disconnect a port from the detected "topology ring" to eliminate the "topology loop" to maintain the integrity of the topology tree in the network.





7. Full Duplex





Full duplex ports can send and receive data at the same time, but this requires both the switch and the attached device to support Full-duplex mode of operation. Switches with FULL-DUPLEX capabilities achieve high throughput (twice times the single mode port throughput), avoid collisions, break the CSMA/CD link length limit, and limit the length of communication links to physical media only.　 In addition, switch ports are best able to achieve full/half duplex automatic conversion.





8. High-speed Port integration




The
switch provides high-bandwidth "pipes" (fixed ports, optional modules, or multilink tunnels) that meet the exchange requirements of switch traffic and superior backbone. Prevent backbone communication bottlenecks. such as FDDI, ATM, G-bit optical modules.





9. Maximum number of VLANs





This parameter reflects the maximum number of VLANs that a single device can support, and the maximum number of VLANs that the switch can support (over 1024) is sufficient to meet the needs of the general enterprise. VLAN partitioning should conform to the 802.1Q standard.





10. Extensibility Configuration





The number of rack slots, expansion slots, maximum number of stackable, 10/100/1000m Ethernet ports, maximum number of ATM ports, maximum number of SONET ports, maximum FDDI port number, maximum power supply, and many other hardware indicators will directly reflect the expansion of the switch and the interconnection with other backbone network equipment.





has different requirements for different users in the selection, such as implementing access Control (ACLs) on the data stream, service quality assurance (QoS), bandwidth management, and various control and service policies, supported packet filtering, load balancing, and the level of support for various routing protocols in the three-tier switch, and so on.





in short, users in the network planning design and selection of switches should carefully examine the various functions of the switch, especially with the rapid exchange of technology, more and more switches to integrate the new features of other network devices, with its superior performance price than become a new favorite user. Especially in some industries users of the switch to choose more cautious and comprehensive.





This article only unifies the work actual and the common project case to make a brief introduction to the switch choice, hoped plays the role, for the general readers in the design network, chooses and uses the broadband switch to play the certain help function.