Cisco 7609-S Software Configuration Guide - Page 324

Enabling PortFast Chapter 21 Configuring Optional STP Features • You cannot enable loop guard on PortFast-enabled ports. • You cannot enable loop guard if root guard is enabled. Loop guard interacts with other features as follows: • Loop guard does not affect the functionality of UplinkFast or BackboneFast. • Enabling loop guard on ports that are not connected to a point-to-point link will not work. • Root guard forces a port to be always designated as the root port. Loop guard is effective only if the port is a root port or an alternate port. You cannot enable loop guard and root guard on a port at the same time. • 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. Note 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. Enabling PortFast Caution Use PortFast only when connecting a single end station to a Layer 2 access port. Otherwise, you might create a network loop. To enable PortFast on a Layer 2 access port, perform this task: Step 1 Step 2 Step 3 Command Router(config)# interface {type1 slot/port} | {port-channel port_channel_number} Router(config-if)# spanning-tree portfast Router(config-if)# spanning-tree portfast default Purpose Selects a port to configure. Enables PortFast on a Layer 2 access port connected to a single workstation or server. Enables PortFast. 21-8 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08