Ethernet: Virtual Switch System (VSS)

To provide high availability and redundancy on an ethernet LAN, network administrators will usually try to add additional hardware to create multiple layer 2 paths traffic can failover to.

Redundant connections and port density to support redundancy can quickly become a problem with more devices added to a switching fabric.  One method to avoid over complication and network design hurdles is to use a virtual switching technology as a solution.

VSS combines multiple devices into a single logical network element.  VSS manages redundant links in such a fashion that they will be seen by external devices as a single port channel.

This simplifies network configuration and operation by reducing the number of layer 3 routing neighbors and simultaneously providing a loop free layer 2 topology.


VSS interacts with with access and core networks as if it were a single switch.

VSS operates within a role-based model, where one device participating in VSS will be the ‘Active’ switch and the other will be the ‘Standby’ switch.

VSS Active switches controls the VSS running the layer 2 and 3 control protocols for the switching modules on both switches.  the active switch also provides management functions for the VSS such as module online insertion and removal and the console interface.

the VSS Active AND standby switch performs packet forwarding for ingress data traffic on their locally hosted interfaces, however the VSS standby switch also sends control traffic to the VSS active switch for processing.

To act as one virtual network element, the two switches need a method to share control information and data traffic … VSS does this by using what’s called Virtual Switch Link (VSL).  VSL is typically implemented via etherchannel for link redundancy and can support up to eight links in a bundle.  control and management traffic are given higher priority across this link in an effort to ensure the the control and management traffic is never discarded.




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