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STP may cause temporary loss of network connectivity when a failover or failback event occurs (1003804)

Symptoms

In a switched network environment which uses Spanning Tree Protocol (STP), you may experience the following symptoms:

  • An ESXi or ESX host temporarily loses network connectivity when a failover or failback event occurs
  • Virtual machines temporarily lose network connectivity when a failover or failback event occurs
  • A VMware High Availability (HA) isolation event occurs after one of the teamed NICs of the COS is unplugged and plugged in to a different port

Resolution

STP is used to accomplish a loop-free environment. Every time a port state goes up, STP calculation occurs. As the result of the calculation, the switch ports are either set to a forwarding or blocking state to prevent a traffic loop.
 
STP topology convergence has four states:
  • Blocking
  • Listening
  • Learning
  • Forwarding

When STP convergence is initiated it forces all of the physical switches in the STP domain to dump their forwarding tables and relearn the STP topology and all MAC addresses. This process can take between 30-50 seconds. During this time, no user data passes through the port. Some user applications can time out during this period. Connectivity is restored when the STP domain completes this convergence.  

There are two ways to prevent the 30-50 second loss of connectivity during STP convergence: 
  • Set STP to Portfast on all switch ports that are connected to network adapters on an ESXi/ESX host.

    Portfast allows the ports to immediately be set back to the forwarding state and prevents the link state changes that occur on ESX/ESXi hosts from affecting the STP topology.  Setting STP to Portfast prevents the 30-50 second loss of network connectivity.

    The command to set STP to Portfast depends on the model of the switch.     As the command will be different from model to model and vendor to vendor, contact your physical switch vendor for more detailed information on how to configure the same.

    Examples:

    To set STP to Portfast on a Cisco switch, run the command:

    spanning-tree portfast (for an access port)
    spanning-tree portfast trunk (for a trunk port)

    To set STP to Portfast on a Dell switch, run the command:

    spanning-tree portfast

    HP switches use a feature called admin-edge-port, which works the same way as Portfast or RSTP.

    To enable admin-edge-port, run the command:

    spanning-tree port-list admin-edge-port
  • Disable STP

    VMware does not typically recommend that you disable STP. However, to prevent this issue from occurring, it may be necessary to disable STP. Before you disable STP, contact your switch vendor.

    The command to disable STP depends on the switch. Contact your switch vendor for more detailed information.

    Example: 

    To disable STP on a Nortel switch, run the command:

    config ethernet stg stp disable

Additional Information

Rapid Spanning Tree Protocol has replaced Spanning Tree Protocol. For more information, see the Cisco white paper Understanding Rapid Spanning Tree Protocol (802.1w).
 
Note: The preceding link was correct as of June 6, 2012. If you find the link is broken, provide feedback and a VMware employee will update the link.

Tags

connect-esx  connect-vm  manage-esx  network-failure  no-network-connectivity  physical-switch  stp-physical-switch

See Also

Update History

06/27/2011 - Added link to the Cisco white paper on Rapid Spanning Tree Protocol 07/10/2012 - Updated for ESXi 5.x 02/05/2013 - Added HP command for admin-edge-port

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