Stacking refers to connecting multiple switches that support stacking features through stacking cables, and logically virtualize them into a switching device, which participates in data forwarding as a whole. Stacking is a horizontal virtualization technology widely used at present, which has the functions of improving reliability, expanding the number of ports, increasing bandwidth, and simplifying networking.

Why do we need to stack?

Traditional campus networks use equipment and link redundancy to ensure high reliability, but their link utilization is low and network maintenance costs are high. Stacking technology virtualizes multiple switches into one switch to simplify network deployment and reduce network maintenance workload. Stacking has many advantages:

Improve reliability

A redundant backup is formed between multiple member switches of the stacking system. As shown in the figure below, Switch A and Switch B form a stacking system, and Switch A and Switch B back up each other. When Switch A fails, Switch B can take over Switch A to ensure the normal operation of the system. In addition, the stacking system supports link aggregation across devices and can also implement link redundancy backup.

Expand the number of ports

As shown in the figure below, when the number of connected users increases to the extent that the port density of the original switch cannot meet the access requirements,new switches can be added to form a stacking system with the original switch to expand the number of ports.

Increase bandwidth

As shown in the figure below, when you need to increase the upstream bandwidth of the switch, you can add a new switch and the original switch to form a stacking system, and configure multiple physical links of the member switches into an aggregation group to increase the upstream bandwidth of the switch.

Simplify networking

As shown in the figure below, multiple devices in the network form a stack, which is virtualized into a single logical device. The simplified networking no longer needs to use MSTP and other ring-breaking protocols, which simplifies the network configuration. At the same time, it relies on cross-device link aggregation to achieve rapid switching when a single device fails and improve reliability.

Long distance stacking

As shown in the figure below, users on each floor connect to the external network through corridor switches. Now the corridor switches that are far apart are connected to form a stack, which is equivalent to only one access device in each building, and the network structure becomes simpler . Each building has multiple links to reach the core network, and the network becomes more robust and reliable. The configuration of multiple corridor switches is simplified into the configuration of a stacking system, which reduces management and maintenance costs.

How to build a stack?

Before introducing how the stack is established, let’s first introduce the related concepts used in the process of stack establishment.

Master,Standby, and slave switch

All single switches in a stack system are called member switches, and can be divided into three roles according to their different functions:

Master switch (Master): The master switch is responsible for managing the entire stack. There is only one main switch in the stacking system.

Standby switch (Standby): The standby switch is the backup switch of the main switch. There is only one standby switch in the stacking system. When the main switch fails, the standby switch will take over all the services of the original main switch.

Slave switch (Slave): The slave switch is used for service forwarding. There can be multiple slave switches in the stacking system. The greater the number of slave switches, the greater the forwarding bandwidth of the stacking system.Except for the master switch and the standby switch, all other member switches in the stack are slave switches. When the standby switch is unavailable, the slave switch assumes the role of the standby switch.

The master switch, standby switch and slave switch can all forward business traffic. Adding, removing, or replacing stack member switches may cause changes in the roles of stack members.

Stack ID

The stack ID is used to identify the stack member switch and is the slot ID of the member switch. Each stack member switch has a unique stack ID in the stack system.

Stacking priority

The stack priority is an attribute of member switches, which is mainly used to determine the role of member switches in the role election process. The larger the priority value, the higher the priority, and the higher the priority, the greater the possibility of being elected as the master switch.

Stack establishment process

The process of stack establishment includes the following four stages:

According to network requirements, select stacking cables and connection methods. The physical connection methods supported by different products are different. For S series campus box switches and CloudEngine data center box switches, two connection topologies, chain and ring, are supported. For CloudEngine data center frame switches, SIP port connection and service port connection are supported.

Election of the master switch. After all member switches are powered on, the stack system starts to elect the master switch. Each member switch in the stacking system has a certain role, among which the main switch is responsible for managing the entire stacking system.

Assign stack ID and elect standby switch. After the election of the master switch is completed, the master switch collects the topology information of all member switches, calculates the stack forwarding table entry based on the topology information, issues it to all member switches in the stack, and assigns stack IDs to all member switches. After that, the election of the standby switch is carried out as the backup switch of the main switch. In addition to the main switch, the switch that completes the device startup first is selected as the backup switch first.

Synchronize software version and configuration files. After role election and topology collection are completed, all member switches wil automatically synchronize the software version and configuration files of the master switch.

The stack system has the function of automatically loading system software. The member switches to be formed into the stack do not need to have the same software version, but only need to be compatible between the versions. When the software version of the standby switch or the slave switch is inconsistent with the master switch, the standby or slave switch will automatically download the system software from the master switch, then restart with the new system software, and rejoin the stack.

The stacking system has a configuration file synchronization mechanism. The master switch saves the configuration file of the entire stacking system and performs configuration management of the entire stacking system. The standby switch or the slave switch will synchronize the configuration file of the master switch to this switch and execute it to ensure that multiple devices in the stack can work in the network like one device, and after the master switch fails, the remaining switches can still Perform various functions normally.