StackWise Virtual

Cisco presented the new feature named “StackWise virtual” supported by the IOS XE Denali in the 3850 switch series and later, in the new Cisco Catalyst 9500 family.

StackWise Virtual is also supported on all Cisco Catalyst 9500 high performance models: C9500-24Y4C, C9500-48Y4C, C9500-32QC, C9500-32C.

Similarly to the old Virtual Switching System (VSS), the stackwise virtual allows the clustering of two chassis together into a single, logical entity, to allow high availability, scalability, management and maintenance.

Cisco® Catalyst® 9000 platform StackWise® Virtual technology allows the clustering of two physical switches together into a single logical entity. The two switches operate as one; they share the same configuration and forwarding state. This technology allows for enhancements in all areas of network design, including high availability, scalability, management, and maintenance

This capability enables a loop-free Layer 2 network topology, since the StackWise Virtual switches are treated as one logical switch for both access and core switches.

let’s break down the StackWise Virtual architecture.

Control Plane: Unified CP, StackWise Virtual Link (SVL) to join the two switches together.
Data Plane: Active/Active DP, both switches are capable of forwarding the traffic locally
SV Components:

Some caveats about StackWise Virtual Switching: When deploying StackWise Virtual, ensure that VLAN ID 4094 is not used anywhere on the network. All inter-chassis system control communication between stack members is carried over the reserved VLAN ID 4094 from the global range.

All the switches in Cisco StackWise Virtual must be running the same StackWise Virtual domain.
All the switches in Cisco StackWise Virtual solution must be of the same switch model.
All the switches in Cisco StackWise Virtual solution must be running the same license level.
All the switches in Cisco StackWise Virtual must be running the same software version.

StackWise Virtual domain ID: Use any range from 1 through 255. No conflict in network.
Switch ID: Supported range is 1 through 9. Renumber if needed to start in ascending number.
Switch priority: Default is 1. Recommended to increase priority for specific stack member if prefer to boot up in ACTIVE role.
Stack MAC address: Default is infinite. Recommended to keep default settings.

The StackWise Virtual switch also simplifies the Layer 3 network topology by presenting itself as one logical switch, thus reducing the number of routing peers in the network.

StackWise Virtual link : it carries data traffic and in-band control traffic between two switches. All the frames that are forwarded over the StackWise Virtual link are encapsulated with a special StackWise Virtual Header (SVH). The data is sent over a StackWise Virtual link under the following circumstances:

Laer 2 traffic flooded over a VLAN.
Pckets processed by software and not by hardware!
System data, such as NetFlow export data and SNMP data.
Layer 3 protocol packets.
Spanning Tree Protocol: the standby switch redirects the STP BPDUs across the Virtual link to active switch.

NOTE: The active and standby switches support hardware forwarding that will individually perform the desired lookups and forward the traffic on local links to uplink neighbors. Hardware forwarding is distributed across both members on Cisco StackWise Virtual thanks to the CPU of the active switch that sends Forwarding Information Base (FIB) updates to the standby switch.

Dual-Active-Detection link : If the standby switch detects a complete loss of the StackWise Virtual link, it assumes the active switch has failed and will take over as the active switch. However, if the original StackWise Virtual active switch is still operational, both the switches will now be Cisco StackWise Virtual active switches. This situation is called a dual-active scenario. To avoid this destructive scenario you need to implement the dual-active-detection link; there are two methods to detect dual active scenario: fast-hello or enhanced PAgP.

in case of dual-active condition (stackwise virtual link down) the active switch shuts down all of its non-SVL interfaces to remove itself from the network and the standby switch takes over Active Supervisor role. To recover from Dual Active condition (assuming stackwise virtual link is operational again), you have to reboot switch that is in a recovery mode (ex-Active Supervisor).

Implementing StackWise Virtual

Config for Switch-1