Cisco WS-C2980G-A Software Guide - Page 164

Configuring PortFast Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop PVID-inconsistent ports. If the port is already blocked by loop guard, misconfigured BPDUs received on the port make loop guard recover, but the port is moved into the type-inconsistent state or PVID-inconsistent state. • In high-availability switch configurations, if a port is put into the blocked state by loop guard, it remains blocked even after switchover to the redundant supervisor engine. The newly activated supervisor engine recovers the port only after receiving a BPDU on that port. • Loop guard uses the ports known to spanning tree. Loop guard can take advantage of logical ports provided by the Port Aggregation Protocol (PAgP). However, to form a channel, all the physical ports grouped in the channel must have compatible configurations. PAgP enforces uniform configurations of root guard or loop guard on all the physical ports to form a channel. These caveats apply to loop guard: - Spanning tree always chooses the first operational port in the channel to send the BPDUs. If that link becomes unidirectional, loop guard blocks the channel, even if other links in the channel are functioning properly. - If a set of ports that are already blocked by loop guard are grouped together to form a channel, spanning tree loses all the state information for those ports and the new channel port may obtain the forwarding state with a designated role. - If a channel is blocked by loop guard and the channel breaks, spanning tree loses all the state information. The individual physical ports may obtain the forwarding state with the designated role, even if one or more of the links that formed the channel are unidirectional. • You can enable UniDirectional Link Detection (UDLD) to help isolate the link failure. A loop may occur until UDLD detects the failure, but loop guard will not be able to detect it. • Loop guard has no effect on a disabled spanning tree instance or a VLAN. Configuring PortFast The following sections describe how to configure PortFast on the switch. Enabling PortFast on an Access Port 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. To enable PortFast on a switch port, perform this task in privileged mode: Step 1 Step 2 Task Enable PortFast on a switch port connected to a single workstation, switch, or server. Verify the PortFast setting on a switch port. Command set spantree portfast mod_num/port_num enable | disable show spantree [mod_num/port_num] [vlan] Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 8-8 78-15486-01