Switch cascading and stacking
The cascading expansion mode is the most conventional and direct expansion method. Some early-built networks use hubs (HUBs) as cascading devices. Because hubs were already quite expensive at that time, most enterprises could not choose switches as cascade equipment. That is because the access requirements of most workgroup users are generally connected from a port on the hub to the hub. In this way, the access capability has been greatly improved, but due to some interference and human factors, the overall performance is very low, only to meet the needs of multiple ports, there is no time to consider the forwarding switching function.
The current cascade expansion mode comprehensively considers the forwarding performance and port attributes of different switches. Through a certain topology design, it can easily achieve multi-user access. The typical structure of the cascade mode is shown in Figure 1.
The cascade connection mode is the most ideal way to form a large-scale LAN. It can comprehensively utilize various topology design technologies and redundancy technologies to achieve a hierarchical network structure, such as designing redundancy through dual-homing and other topology structures, and implementing redundancy and redundancy through Link AggregaTIon technology. Up Link's bandwidth expansion, these technologies are now very mature, widely used in various local area networks and metropolitan area networks.
The cascade connection mode uses common Ethernet ports for interconnection between layers, such as 100M FE ports, GE ports, and emerging 10GE ports.
Cascading mode is the mainstream technology in the application of Ethernet expansion ports. It uses a unified network management platform to achieve unified management of the entire network equipment, such as topology management and fault management. The cascading mode also faces challenges. When there are many cascading layers and there is a large convergence ratio between layers, there will be a certain delay due to more exchanges and buffers between edge nodes. . The solution is to aggregate upstream ports to reduce the convergence ratio, improve the performance of the upper-end device, or reduce the level of cascading. In cascading mode, in order to ensure the efficiency of the network, it is generally recommended that the number of layers should not exceed four layers. If the network edge node has a port that is extended by a broadcast Ethernet device such as HUB, because it is a pass-through mode, there is no switching, and it is not included in the hierarchy, but it should be noted that the CSMA / CD mechanism for HUB operation The loopback caused by the conflict may cause a greater network performance impact than the switch cascading.
The cascade connection mode is an inevitable choice for constructing a structured network. The cascade connection uses general-purpose cables (optical fibers), and each component can be placed at any location, which is very conducive to integrated wiring.
Stacking technology expansion
Stacking technology is another type of technology that is currently used more for expansion ports on Ethernet switches, and is a non-standardized technology. Various vendors do not support hybrid stacking. The stacking mode is defined by each vendor and does not support topology. At present, there are two main stacking modes: daisy chain mode and star mode. The biggest advantage of stacking technology is to provide simplified local management and manage a group of switches as an object.
Daisy chain stacking
Daisy-chain stacking is a stacking technology based on a cascade structure. There are no special requirements for switch hardware. Through relatively high-speed port concatenation and software support, a multi-switch stacking structure is finally built. Redundancy can be achieved to a certain extent. However, in terms of switching efficiency, the same cascade mode is at the same level. Daisy-chain stacking usually has a mode using one high-speed port and two high-speed ports. The structure of the two is shown in Figure 2. In the mode of using a high-speed port (GE), the upstream and downstream of the same port are sent and received separately to form a ring structure. The data exchange between any two member switches needs to go around the ring and pass through the switch ports of all switches. The efficiency is low, especially when there are many stacking layers, stacking ports will become a serious system bottleneck. Use two high-speed ports to implement daisy-chain stacking. As more high-speed ports are occupied, you can choose to implement ring redundancy. Compared with the cascade connection mode, the daisy-chain stacking mode does not have topology management and generally cannot be distributed. It is suitable for single-node organizations that require high-density ports and can be used at the edge of the network.
The daisy chain structure needs to eliminate the broadcast storm caused by the loop. Under normal circumstances, at any time, a slave switch in the loop can only reach the master switch through a high-speed port (that is, a high-speed port cannot share the cost. The upstream data pressure of the switch) needs to be exchanged through all upstream switches (see Figure 3).
Daisy chain stacking is a type of simplified stacking technology. It is mainly an expansion port technology that provides centralized management. It does not improve the forwarding efficiency between multiple switches (the efficiency will be much lower than the cascade connection mode in single-port mode). Hardware needs to provide more high-speed ports, while software realizes the redundancy of UP LINK. The number of layers in a daisy-chain stack should generally not exceed four layers, requiring all stacking group members to be placed close enough (generally on the same rack).
Star stacking technology is an advanced stacking technology. For switches, it is necessary to provide an independent or integrated high-speed switching center (stacking center). All stacking hosts are stacked through a dedicated (also can be a universal high-speed port) high-speed stacking. The port is uplinked to a unified stacking center. The stacking center is generally a hardware switching unit based on a dedicated ASIC. According to its switching capacity, the bandwidth is generally between 10 and 32G. Its ASIC switching capacity limits the number of stacked layers (see Figure 4) .
Star stack
Star stacking technology reduces the number of stages of all stack group member switches to the stacking center Matrix to one level. The forwarding between any two end nodes requires only three exchanges, and the forwarding efficiency is on the edge of the first level connection mode. The node communication structure is the same, so compared with the daisy chain structure, it can significantly improve the data forwarding rate between stack members, and at the same time, provide a unified management mode. A group of switches can be used as a single node in network management appear.
Star stacking mode is suitable for single-node LANs that require high-efficiency and high-density ports. Star stacking mode overcomes the high latency impact of daisy-chain stacking mode multi-layer forwarding, but it needs to provide a high-bandwidth Matrix, which costs more, and Matrix interfaces are generally not universal. Neither the stacking center nor the stacking ports of member switches can be used to connect other network devices. The use of highly reliable and high-performance Matrix chips is the key to star stacking. The bandwidth of general stack cables is between 2G and 2.5G (bidirectional), which is slightly higher than that of GE. The higher part is usually only used for member management, so the effective data bandwidth is basically similar to GE. However, due to the special bus technology involved, the cable length generally cannot exceed 2m. Therefore, in the star stack mode, all switches need to be limited to one rack.
It can be seen that the traditional stacking technology is a centralized management port expansion technology, which cannot provide topology management, has no international standards, and has poor compatibility. However, in a single-node LAN that requires a large number of ports, star stacking can provide excellent forwarding performance and convenient management features. Cascading is the foundation of a network. It can utilize various topologies and redundancy technologies flexibly. When there are too many layers, careful design is required. For networks with few cascade levels, the cascade mode can provide the best performance. For example, in a network point that needs to be expanded to twice as many ports, it needs to be exchanged three times between the edges of the star stack, the cascade mode and the daisy chain stack need to be exchanged twice, and the star stack mode requires a larger investment. Mode needs to occupy more high-speed ports, and ordinary cascade connection has become the most economical and efficient way to form. In addition, you can also use the existing switching equipment without repeated investment, but these two equipment need to be managed separately (see Figure 5).
The application of traditional stacking technology is often limited by the geographical location, and often needs to be placed in the same rack. In the application of high-density ports, it will cause difficulties in wiring. Therefore, major manufacturers have actively sought to support distributed stacking technology. Currently, Huawei's Quidway S series Ethernet switch products and Cisco series Ethernet switch products both provide cluster management mode. Quidway S series Ethernet switches use Huawei's unified VRP operating system and unified iManager network management system. The network management system supports the Chinese interface, adopts standard protocols and open technologies, and is fully compatible with mainstream network management platforms. Quidway S series Ethernet switches take full advantage of product development advantages under the guidance of the design ideas of Huawei ’s Layer 2 switching full line rate, Layer 3 switching full line rate, business switching full line rate and QoS service full line rate. It meets the needs of broadband metropolitan area networks and enterprise networks in terms of product system design, expansion capabilities, and rich business features, and can provide customers with more efficient, safe, and easily scalable customized solutions.
Taking Huawei products (HGMP) as an example, through the support of cluster management mode, centralized configuration and management can be realized on the network constructed by the Quidway series switches through the universal cascade mode. A LAN can be added as a group. For the network management system , A group can be represented as the same device, using one IP address for management, which is equivalent to or even better than the management effect of the previous stacking group. However, as a centralized manifestation of versatility, group member switches can achieve topology design and distributed placement of members within the group, and stack ports can optionally select general ports supported by the device or use port aggregation, allowing users to obtain flexible control of the switching network The ability to stack bandwidth to achieve higher flexibility requirements.
For different environments, the effect of selecting different port expansion modes is inconsistent. In the current situation, the ordinary cascade mode is still the main application means to solve the hierarchical network. The star stack mode is a simple management mode that provides single-node port expansion, and the distributed stack realized through cluster management will be the next generation stack. The main way.
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