Cisco WS-C2980G-A Software Guide - Page 158

Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Understanding How PortFast BPDU Guard Works You can use PortFast on switch or trunk ports connected to a single workstation, switch, or server to allow those devices to connect to the network immediately, instead of waiting for the port to transition from the listening and learning states to the forwarding state. Caution You can use PortFast to connect a single end station or a switch port to a switch port. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops. When the switch powers up, or when a device is connected to a port, the port normally enters the spanning tree listening state. When the Forward Delay timer expires, the port enters the learning state. When the Forward Delay timer expires a second time, the port is transitioned to the forwarding or blocking state. When you enable PortFast on a switch or trunk port, the port is immediately transitioned to the spanning tree forwarding state. Understanding How PortFast BPDU Guard Works To prevent loops from occurring in a network, the PortFast mode is supported only on nontrunking access ports because these ports typically do not transmit or receive BPDUs. The most secure implementation of PortFast is to enable it only on ports that connect end stations to switches. Because PortFast can be enabled on nontrunking ports connecting two switches, spanning tree loops can occur because BPDUs are still being transmitted and received on those ports. The PortFast BPDU guard feature prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port. When the BPDU guard feature is enabled on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs, instead of putting them into the spanning tree blocking state. In a valid configuration, PortFast-configured interfaces do not receive BPDUs. If a PortFast-configured interface receives a BPDU, an invalid configuration exists, such as connection of an unauthorized device. The BPDU guard feature provides a secure response to invalid configurations because the administrator must manually put the interface back in service. Note When enabled on the switch, spanning tree applies the BPDU guard feature to all PortFast-configured interfaces. Understanding How PortFast BPDU Filtering Works BPDU filtering allows you to avoid transmitting BPDUs on PortFast-enabled ports that are connected to an end system. When you enable PortFast on the switch, spanning tree places ports in the forwarding state immediately, instead of going through the listening, learning, and forwarding states. By default, spanning tree sends BPDUs from all ports regardless of whether PortFast is enabled. BDPU filtering is on a per-switch basis; after you enable BPDU filtering, it applies to all PortFast-enabled ports on the switch. Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 8-2 78-15486-01