Campus lan and Wireless lan solution Design Guide

StackWise Virtual Technology 
StackWise Virtual technology combines two Catalyst 9000 Series switches into a single logical network entity 
from the network control plane and management perspectives. Because the two switches operate as one, 
StackWise Virtual enables the creation of a loop-free topology. Spanning-tree treats the StackWise Virtual pair 
as one bridge node, instead of two. StackWise Virtual technology uses SSO / NSF to provide seamless traffic 
failover when one of the switches fails. To neighboring devices a StackWise Virtual domain appears as a single 
logical switch or router. Within a StackWise Virtual domain, one device is designated as the active switch and 
the other is designated as the standby switch. All control plane functions are centrally managed by the active 
switch. From the data-plane and traffic-forwarding perspectives, both switches actively forward traffic. 

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StackWise Virtual Technology 
Figure 20. 
In order to bond the two switches together into a single logical node, special signaling and control information 
must be exchanged between the two switches. To facilitate this information exchange, a dedicated link – the 
StackWise Virtual link (SVL) – is used to transfer both data and control traffic between the peer switches. The 
SVL is formed as an EtherChannel interface of up to eight physical port members. It is recommended to have at 
least two physical port members for StackWise Virtual link resiliency.
Switch Stacks and Cisco StackWise Technology 
Cisco StackWise technology allows up to a maximum of eight switches to be stacked together physically in a 
ring topology to form a single, unified, virtual stack system. The stacking architecture expands form factor, 
switching capacity, port density, and redundancy, as well as providing a distributed data plane with a single 
control and management plane.
StackWise creates a unified control and management plane by electing one switch in the stack as the active 
switch and another switch as the hot-standby. Remaining switches become stack members. To logically 
appear as a single virtual switch, the IOS daemon (IOSd) process on the active switch of the stack centrally 
manages all management plane and network control plane operations with Layer 2 and Layer 3 protocols. This 
information is synchronized with the standby switch of the stack to provide NSF / SSO failover in case the active 
switch fails. To optimize data plane performance by using hardware resources from each Catalyst 9000 Series 
stack member switch, network services such as QoS, security ACLs, and others are distributed and 
programmed to be locally enforced on network ports. The hardware Forwarding Information Base (FIB) is also 
programmed in ASICs across all stack-member switches in the stack ring. 
To optimally forward the traffic within the stack ring, the packet-stripping function is performed on the 
destination switch instead of on the source switch. This mechanism, known as the spatial-reuse forwarding 
mechanism, boosts data plane switching performance in the stack-ring switching architecture.
The following sub-sections discuss the StackWise implementation on Catalyst 9200 and 9300 Series switches.