Virtual Switching System: A Switching Technology


Virtual Switching System :VSS is network system virtualization technology that pools multiple Cisco Catalyst 6500 Series Switches into one virtual switch, increasing operational efficiency, boosting nonstop communications, and scaling system bandwidth capacity to 1.4 Tbps. At the initial phase, a VSS will allow two physical Cisco Catalyst 6500 Series. The VSS is made up of the following:

  • Members of VSS: Cisco Catalyst 6500 Series Switches (up to two switches with initial release) deployed with the Virtual Switching Supervisor 720 10GE
  • Virtual switch link (VSL): 10 Gigabit Ethernet connections (up to eight using EtherChannel) between the members.
  • The 10 Gigabit Ethernet connections have to be one or more of the modules: Switching Supervisor 720 10GE: 10 Gigabit Ethernet uplink ports

VSS enables unprecedented functionality and availability of campus network by integrating network and systems redundancy into a single node.VSS is Cisco validated design. It is Cisco Safe Harbor program tested and certified. It allows us to

  • Maximize network performance
  • Increase network availability
  • Simplify network architecture
  • Reduce administrative burden
  • Support virtualization


  • VSS1440:-

VSS1440 refers to the VSS formed by two Cisco Catalyst 6500 Series Switches with the Switching Supervisor 720-10GE. In a VSS, the data plane and switch fabric with capacity of 720 Gbps of supervisor engine in each chassis are active at the same time on both chassis, combining for an active 1400-Gbps switching capacity per VSS. Only one of the virtual switch members has the active control plane. Both chassis are kept in sync with the interchassis Stateful Switchover (SSO) mechanism along with Nonstop Forwarding (NSF) to provide nonstop communication even in the event of failure of one of the member supervisor engines.

  • Stateful SwitchOver(SSO) mechanism:

A VSS uses interchassis NSF/SSO as the primary mechanism for high availability between the two chassis. One virtual switch member chassis will act as the active virtual switch member, while the other member will be in hot standby state for the control plane. Note that the data planes of both chassis are active and hence forward traffic at full combined capacity of 1440 Gbps. When one of the virtual switch members fails, there is no reconvergence of protocols in the network. The access layer or core layer switches continues to forward traffic because they only detect a link failure in an EtherChannel bundle and hence do not need to reconverge any protocols. No disruption occurs to the traffic flowing through the VSS. The VSS mechanism during switch failure is far superior when comparison with the traditional model where one switch failure results indeterminist convergence of multiple control protocols like STP, HSRP and routing protocol.

  • Multichassis Ether Channel(MEC):

Multichassis EtherChannel (MEC) is a Layer 2 multipathing technology. This form of EtherChannel allows a connected node to terminate the EtherChannel across the two physical Cisco Catalyst 6500 Series Switches that make up the VSS leading to creating simplified loop-free Layer 2 topology. Using MEC in VSS topology results in all links being active and at the same time provides for a highly available topology without the dependency of Spanning Tree Protocol. With the introduction of 12.2(33)SXI, the virtual switching system supports a maximum number of 512 MECs.

  • Virtual Switch Link(VSL):

The connection used for communication between the two chassis. VSLs can be configured with up to eight links between the two switches across any combination of line cards or supervisor ports to provide a high level of redundancy. If for some rare reason all VSL connections are lost between the switch members leaving both the members up, the VSS will transition to the dual active recovery mode. The dual active state is detected rapidly (subsecond) by any of the following three methods:

  • Enhancement to PAgP used in MEC with connecting Cisco switches
  • L3 Bidirectional Forwarding Detection (BFD) configuration on a directly connected link (besides VSL) between switch members or through an L2 link through an access layer switch
  • L2 Fast-Hello Dual-Active Detection configuration on a directly connected link (besides VSL) between switch members (supported with 12.2(33)SXI) In the dual active recovery mode, all interfaces except the VSL interfaces are in an operationally shut down state in the formerly active switch member. The new active virtual switch continues to forward traffic on all links.

Need for VSS: The process of designing a reliable, fast Network Infrastructure is challenged by new business requirements. The need for non-stop communication is becoming a basic starting point for most campus networks.

  • High-bandwidth environments
  • Virtualization (VMs)
  • 10Gbps
  • High-availability environments
  • Minimize network downtime
  • Redundant infrastructure
  • Reduction in number of devices to manage.

Deployment areas for VSS:

  • Campus or data center core/distribution layer
  • Data center access (server connectivity)

Benifits of VSS:

VSS offers superior benefits compared to traditional Layer 2/Layer 3 network design. Benefits can be grouped into following categories:

  • Reduce administrative burden:1 active control plane = 1 logical switch. Using VSS there is a 50% reduction in the number of switches that must be managed. Time to properly prepare for a change window can be drastically reduced. Approximately 60% of network failures are caused by human error.
  • It Reduces the number of times you and I have to touch the switch and we might reduce network failures
  • Maximize network performance: Double the bandwidth

2 active forwarding planes (720Gbps each)

2 x 720Gbps = 1,440Gbps (1.44Tbps)

  • Increase network availability – Multichassis EtherChannel (MEC) allows us to reduce the number of neighbor adjacencies eliminate SPF, DUAL and STP calculations in the event of a – single link failure – VSS chassis failure.
  • Perform IOS and chassis upgrades with minimal disruption.
  • Only one gateway IP address is required per VLAN, instead of the three IP addresses per VLAN used.


Source by Shiv Shankar Pandey