ortfast) is enabled, can cause temporary bridging loops. Use with CAUTION 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 connected 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-tree bpduguard enable command in IOS: Chapter 8: Spanning Tree ht-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. FEXs on Nexus systems have BPDU Guard on by default and should not be removed. This is to prevent you from connecting a switch to the FEX, since they are designed for host connectivity only. 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 port fails. This allows the network to recover in five seconds less. This feature affects all VLANs on the switch. It also sets the bridge priority to 49,152 to ensure that the switch will not become the root bridge. Figure 8-6 shows where UplinkFast would be applied in a simple network. Switches A and B would be either