How to Configure a Cisco Virtual Switching System

What Is a Virtual Switching System?

• A Virtual Switching System (VSS) is a Cisco proprietary technology that interconnects two physical switches and logically makes them one switch with interconnected control and management planes. This architecture simplifies the design process and reduces overhead on the units as they logically process as one unit. VSS optimizes operational efficiency and allows for more scalable bandwidth up to 1.4 Tbps.
• As shown below, the two physical units are logically joined to create one virtual switch. VSS eliminates the need for protocols like HSRP (Hot Standby Routing Protocol) or VRRP (Virtual Router Redundancy Protocol), which are generally put in place to help with redundancy. These protocols still require each switch to have its own control plane, which is where VSS is more optimal.

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• Instead of an Active/Passive relationship, the two switches form more of a Master/Slave relationship, with one switch becoming the primary. The primary switch is then responsible for the configuration of both switches. One switch within the chassis will act as the Active Virtual Switch, while the other sits as a Standby Virtual Switch.
• One of the downsides to HSRP or VRRP is that Spanning Tree will block one of the uplinks from the Access Layer to the Core because it sees the switches as separate entities. VSS eliminates this as it logically combines the two switches and takes advantage of port-channeling, which creates one logical link between the Access and Core, ultimately optimizing bandwidth. Spanning Tree convergence time is also reduced to sub 200ms.
• As shown above, two independent Catalyst 6500 switches become one virtual chassis, essentially doubling the slot capacity from 9 to 18. Adjacent devices in your network will view the VSS-enabled switch as one logical switch (or router).
• The Supervisor Engine on the Active Virtual Switch will manage the control plane, which includes the following:
  • Layer 2 Protocols (STP, LACP, BPDUs, PDUs, etc.)
  • Layer 3 Protocols (EIGRP, OSPF, etc.)
  • Management Functions (SNMP, SSH, etc.)
• The data planes on both switches will stay active as traffic flows physically through both while logically intertwined, the Control plane on the VSS Standby Switch will become active when the VSS Active Switch becomes non-responsive.

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Configure Port Channel

Configuration of VSS

A prerequisite to configuring VSS is enabling SSO for the switches, otherwise, access to the switches may be limited or disabled.

SSO Configuration

VSS Configuration

To complete the conversion, execute the following:

PiVIT-6500SW1 # switch convert mode virtual

To view the configuration:

PiVIT-6500SW1 # show virtual switch

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