Cisco StackWise and StackWise Plus technologies create a unified, logical switching architecture through the linkage of multiple, fixed configuration 3750G and 3750E switches.
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Cisco 3750G switches use StackWise technology, and Cisco 3750E switches can use either StackWise or StackWise Plus. (StackWise Plus is used only if all switches within the group are 3750E switches; whereas, if some switches are 3750E and others are 3750G, StackWise technology will be used.)
Also to prevent excessive wordiness, “StackWise” is used in this section to refer to both StackWise and StackWise Plus technologies, with the exception of explicitly pointing out the differences between the two at the end of this section.
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Cisco StackWise technology intelligently joins individual switches to create a single switching unit with a 32-Gbps switching stack interconnect. Configuration and routing information is shared by every switch in the stack, creating a single switching unit. Switches can be added to and deleted from a working stack without affecting availability.
The switches unite into a single logical unit using special stack interconnect cables that create a bidirectional closed-loop path. This bidirectional path acts as a switch fabric for all the connected switches. Network topology and routing information is updated continuously through the stack interconnect. All stack members have full access to the stack interconnect bandwidth. The stack is managed as a single unit by a master switch, which is elected from one of the stack member switches.
Each switch in the stack has the capability to behave as a master in the hierarchy. The master switch is elected and serves as the control center for the stack. Each switch is assigned a number. Up to nine separate switches can be joined together.
Each stack of Cisco Catalyst 3750 Series switches has a single IP address and is managed as a single object. This single IP management applies to activities such as fault detection, VLAN creation and modification, security, and quality of service (QoS) controls. Each stack has only one configuration file, which is distributed to each member in the stack. This allows each switch in the stack to share the same network topology, MAC address, and routing information. In addition, it allows for any member to immediately take over as the master, if there is a master failure.
To efficiently load balance the traffic, packets are allocated between two logical counter-rotating paths. Each counter-rotating path supports 16 Gbps in both directions, yielding a traffic total of 32 Gbps bidirectionally. When a break is detected in a cable, the traffic is immediately wrapped back across the single remaining 16-Gbps path (within microseconds) to continue forwarding.
Switches can be added and deleted to a working stack without affecting stack availability. (However, adding additional switches to a stack might have QoS performance implications) Similarly, switches can be removed from a working stack without any operational effect on the remaining switches.
Stacks require no explicit configuration but are automatically created by StackWise when individual switches are joined together with stacking cables, as shown in Figure 1. When the stack ports detect electromechanical activity, each port starts to transmit information about its switch. When the complete set of switches is known, the stack elects one of the members to be the master switch, which will be responsible for maintaining and updating configuration files, routing information, and other stack information. This process is referred to as hot stacking.
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Master switch election occurs only on stack initialization or if there is a master switch failure. If a new, more favorable switch is added to a stack, this will not trigger a master switch election, nor will any sort of preemption occur.
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Each switch in the stack can serve as a master, creating a 1:N availability scheme for network control. In the unlikely event of a single unit failure, all other units continue to forward traffic and maintain operation. Furthermore, each switch is initialized for routing capability and is ready to be elected as master if the current master fails. Subordinate switches are not reset so that Layer 2 forwarding can continue uninterrupted.
- StackWise uses source stripping, and StackWise Plus uses destination stripping (for unicast packets). Source stripping means that when a packet is sent on the ring, it is passed to the destination, which copies the packet and then lets it pass all the way around the ring. After the packet has traveled all the way around the ring and returns to the source, it is stripped off of the ring. This means bandwidth is used up all the way around the ring, even if the packet is destined for a directly attached neighbor. Destination stripping means that when the packet reaches its destination, it is removed from the ring and continues no further. This leaves the rest of the ring bandwidth free to be used. Thus, the throughput performance of the stack is multiplied to a minimum value of 64 Gbps bidirectionally. This capability to free up bandwidth is sometimes referred to as spatial reuse.
Note Even in StackWise Plus, broadcast and multicast packets must use source stripping because the packet might have multiple targets on the stack. - StackWise Plus can locally switch; StackWise cannot. In StackWise Plus, packets originating and destined to ports on the same local switch will not have to traverse the Stack ring, which results in more efficient switching. In contrast, in StackWise, because there is no local switching and because there is source stripping, even locally destined packets must traverse the entire stack ring.
- StackWise Plus can support up to two Ten Gigabit Ethernet ports per Cisco Catalyst 3750-E.
Finally, both StackWise and StackWise Plus can support Layer 3 NSF when two or more nodes are present in a stack. NSF is discussed in the following section, along with SSO.
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