FlexStack is the name of the stacking technology used by the Cisco Catalyst 2960-S Series of fixed Ethernet switches. FlexStack is specific to Cisco Catalyst 2960-S switches, and only Cisco Catalyst 2960-S Series Switches use FlexStack. FlexStack-Plus is the name of the stacking technology used by the Cisco Catalyst 2960-X and 2960-XR Series of fixed Ethernet switches. FlexStack-Plus is specific to the 2960-X and 2960-XR Series of switches. Both FlexStack and FlexStack-Plus require a specific external module in order to stack together. In addition to theFlexStack and FlexStack-Plus modules , a special FlexStack cable is also required to interconnect the modules. There are two different modules for FlexStack. The FlexStack module (PID: 2960S-STACK) is for 2960-S, and the FlexStack-Plus module (PID: 2960X-STACK) is for 2960-X and 2960-XR. These two modules are different sizes and cannot be inserted into the wrong 2960 model. Each FlexStack module supports two FlexStack ports. The FlexStack module is inserted into the rear of the Cisco Catalyst 2960 switch. Two FlexStack cables, inserted into the FlexStack module, provide data path redundancy for traffic flowing across the stack. Using FlexStack cables, the physical members of the stack form a ring, providing the built-in redundant data path for each member of the stack. FlexStack and FlexStack-Plus share the same stack cable types. You do not need different FlexStack cable types for FlexStack and FlexStack-Plus. FlexStack and FlexStack-Plus are optional. In order for a 2960 switch to stack, a separate FlexStack or FlexStack-Plus module (depending upon your exact 2960 model) must be inserted in the rear of the switch. Without this module, stacking cannot function. Figure 3 shows a FlexStack-Plus module to be inserted into the rear of a Cisco Catalyst 2960-X switch. All FlexStack module types are hot swappable. Cisco Catalyst 2960 switches running FlexStack and FlexStack-Plus use a hop-by-hop method of transferring Ethernet packets across the stack. Packets traverse the stack by going from one member to another over the FlexStack links until the packet reaches its destination. This is a behavior similar to that of multiple standalone Ethernet switches forwarding packets from one switch to another. Switches in a stack communicate with each other using the FlexStack protocol. The FlexStack protocol allows the FlexStack members to behave as one logical switch. The FlexStack protocol runs on each member. The FlexStack protocol is used by the master switch to manage the other members. The master tracks the presence of each member, the member’s availability, the Cisco IOS® Software image, and the status of each member’s FlexStack connections. The FlexStack protocol is the same for FlexStack on the 2960-S models and the 2960-X and XR models. Because it is the same protocol for all models of 2960, the different 2960 model types can be stacked together.