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Hosts connected to a Catalyst switch are disconnected for a few seconds when a new switch is introduced in the network

Core issue

Whenever a forwarding switch port detects a topology change, it goes into the blocking state to reconfigure the tables regarding the spanning tree topology change. These are among the reasons for topology changes:

  • The addition of a new switch to the network
  • The removal of an old switch
  • A configuration change or hardware replacement by an administrator.

Spanning Tree Protocol (STP) configures meshed topology into a loop-free, tree-like topology. When the link on a bridge port goes up, STP calculation occurs on that port. The result of the calculation is the transition of the port into forwarding or blocking state. The result depends on the position of the port in the network and the STP parameters. This calculation and transition period usually takes about 30 to 50 seconds. At that time, no user data passes through the port. Some user applications can time out during the period.

Resolution

In order to allow immediate transition of the port into forwarding state, enable the STP PortFast feature. PortFast immediately transitions the port into STP forwarding mode upon linkup. The port still participates in STP. So if the port is to be a part of the loop, the port eventually transitions into STP blocking mode.

If a user connects a Workstation or a server with a single Network Interface Card (NIC) to a switch port, this connection cannot create a physical loop. These connections are considered leaf nodes. There is no reason to make the Workstation wait 30 seconds while the switch checks for loops when the Workstation cannot cause a loop.

Note: The PortFast feature should never be used on switch ports that connect to other switches, hubs, or routers. These connections may cause physical loops and it is very important that STP go through the full initialization procedure in these situations. A spanning tree loop can bring the network down.

For more information, refer to the document Using  PortFast and Other Commands to Fix Workstation Startup Connectivity Delays.

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