CCIE 400-101: Layer 2 Technologies - VSS, StackWise Concepts


  • VSS will allow up to two physical Cisco Catalyst 6500 or 4500 switches to operate as one virtual switch. 
  • The two physical chassis do not need to be identical in the type of modules installed or even type of chassis. 
  • The supervisors need to be of same type with same software version.
  • 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. 
  • The data planes of both chassis are active and hence perform packet forwarding at full combined capacity.  
  • The VSS active switch controls the VSS, running the Layer 2/3 control protocols for the switching modules on both switches. 
  • The virtual switch link (VSL) connects the two VSS devices. 
  • The VSL is a special link that carries control and data traffic between the active and standby switches. 
  • VSS effectively simplifies or even eliminates the need for common Layer 2/3 protocols like STP or FHRPs. (Note: STP should still be enabled to protect against misconfiguration.)  
  • 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 
  • The VSS mechanism during switch failure is far superior in comparison with the traditional model where one switch failure results indeterminist convergence of multiple control protocols like STP, HSRP and routing protocol. 
  • Multichassis EtherChannel (MEC) is an EtherChannel whose member ports can be distributed across the member switches in a VSS.


  • Individual switches intelligently join to create a single switching unit. 
  • The switches are united into a single logical unit using special stack interconnect cables that create a bidirectional closed-loop path. 
  • This bidirectional path acts as a switch fabric for all the connected switches.  
  • Network topology and routing information is updated continuously through the stack interconnect. 
  • The stack is managed as a single unit by a master switch, which is elected from one of the stack member switches. 
  • Stack master election:
    1. User priority - The network manager can select a switch to be master.
    2. Hardware and software priority - This will default to the unit with the most extensive feature set.
    3. Default configuration - If a switch has preexisting configuration information, it will take precedence over switches that have not been configured.
    4. Uptime - The switch that has been running the longest is selected.
    5. MAC address - Each switch reports its MAC address to all its neighbors for comparison. The switch with the lowest MAC address is selected. 
  • The master switch is elected and serves as the control center for the stack.  
  • Both the master and member switches act as forwarding agents.  
  • Up to nine separate switches can be joined together. 
  • The stack can have switches added and removed without affecting stack performance.
  • The stack shares an IP address and is managed as a single object. 
  • The stack has only one configuration file, which is distributed to each member in the stack.


Virtual Switching System (VSS) Q&A
Cisco StackWise and StackWise Plus Technology