tfast Cat-3550(config-if)# Chapter 8: Spanning Tree On a CatOS switch, the command to enable PortFast is set spantree portfast <mod/ port> enable. Executing this command will also result in a nice message about the dangers of PortFast: Catos-6509: (enable) set spantree portfast 3/10 enable Warning: Connecting Layer 2 devices to a fast start port can cause temporary spanning tree loops. Use with caution. Spantree port 3/10 fast start enabled. To disable PortFast, use the same command with disable instead of enable: Catos-6509: (enable) set spantree portfast 3/10 disable Spantree port 3/10 fast start disabled. BPDU Guard Ports configured for PortFast should never receive BPDUs as long as they are connected to devices other than switches/bridges. If a PortFast-enabled port is con- nected to a switch, a bridging loop will occur. To prevent this, Cisco developed a feature called BPDU Guard. BPDU Guard automatically disables a port configured for PortFast in the event that it receives a BPDU. The port is not put into blocking mode, but is put into the ErrDisable state. Should this happen, the interface must be reset. BPDU Guard is enabled with the spanning- bpduguard enable command in Cat-3550(config-if)# spanning-tree bpduguard enable To disable this feature, change the enable keyword to disable. In CatOS, use the set spantree bpdu-guard <mod/port> enable (or disable) command: Catos-6509: (enable) set spantree bpdu-guard 3/10 enable Spantree port 3/10 bpdu guard enabled. UplinkFast UplinkFast is a feature designed for access-layer switches. These switches typically have links to other switches to connect to the distribution layer. Normally, when the link on the designated port fails, a port with an alternate path to the root bridge is cycled through the spanning tree listening and learning states until it returns to the forwarding state. Only then can the port pass traffic. This process can take 45 seconds or more. UplinkFast allows a blocked uplink port to bypass the listening and learning states when the designated 