Cisco VSS

Virtual Switching System – VSS


What is a Virtual Switching System (VSS)?

  • A VSS is network system virtualization technology that pools multiple Cisco Catalyst 6500 Series switches into one virtual switch.
  • It helps in increasing operational efficiency, boosting nonstop communication & scaling system bandwidth capacity up to 1.4 Tbps
  • We can manage 2 Cisco Catalyst 6509 chassis as a single 18 slot chassis once VSS is enabled.
  • VSS eliminates spanning tree & provides increased bandwidth.


Supported Platform:

  • Catalyst 6500 & Catalyst 4500 series
  • Virtual Switching Supervisor 720-10GE (VS-S720-10GE-3C and VS-S720-10GE-3CXL) with IOS 12.2(33)SXH1 & IP Base
  • Supervisor 2T (VS-S2T-10G and VS-S2T-10G-XL) with IOS 12.2(50) SY and IP Base

The peer switches which needs to form the VSS should be identical to each other with respect to both hardware & software version.

Cisco VSS – Logical Representation:


Cisco VSS Architecture:

  • Virtual Switching System allows the combination of 2 switches into a single, logical network entity from the network control plane & management perspective.
  • VSS uses Cisco IOS Stateful Switchover (SSO) & Non-Stop Forwarding (NSF) to provide a single logical switching & routing entity.
  • To neighboring devices the VSS appears as a single logical switch.
  • From data plane & traffic-forwarding perspective both switches in the Virtual Switching System actively forward traffic.
  • From Control plane perspective one switch will be active & the other will be standby.


Key Features:

  • 1 Active redundant control plane
  • 1 Config
  • Single point of management
  • 2 active data planes
  • Standby switch is essentially a set of additional line cards
  • Control messages &  Data frames flow between active and standby via VSL (can be seen as backplane extension)
  • Special encapsulation on VSL frames to carry additional information


  • VSL is a dedicated link which bonds the 2 chassis together into a single logical node.
  • This link is responsible for transferring both data & control traffic between the peer chassis.
  • The VSL is formed as a Cisco Ether Channel interface which carries VSS control traffic & normal data traffic.
  • Up to 8 physical ports can form the ether channel for forming the VSL link.

Sample Configuration: