Cisco CCNA Study Guide - Router Alley

CCNA Study Guide v2.62 – Aaron Balchunas
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STP PortFast
PortFast allows switch ports that connect a host device (such as a printer or
a workstation), to bypass the usual progression of STP states. Theoretically,
a port connecting to a host device can never create a switching loop. Thus,
Port Fast allows the interface to move from a blocking state to a forwarding
state immediately, eliminating the normal 30 second STP delay.
To configure PortFast on an interface:
Switch(config)# int fa0/10
Switch(config-if)# spanning-tree portfast
To enable PortFast globally on all interfaces:
Switch(config)# spanning-tree portfast default
PortFast should not be enabled on switch ports connecting to another
hub/switch, as this may result in a loop. Note that PortFast does not disable
STP on an interface - it merely speeds up the convergence.
PortFast additionally reduces unnecessary BPDU traffic, as TCN BPDU’s
will not be sent out for state changes on a PortFast-enabled interface.
STP UplinkFast
Switches can have multiple uplinks to other upstream switches. If the
multiple links are not placed in an EtherChannel, then at least one of the
ports is placed into a blocking state to eliminate the loop.
If a directly-connected interface goes down, STP needs to perform a
recalculation to bring the other interface out of a blocking state. As stated
earlier, this calculation can take from 30 to 50 seconds.
UplinkFast allows the port in a blocking state to be held in standby-mode,
and activated immediately if the forwarding interface fails. If multiple ports
are in a blocking state, whichever port has the lowest Root Path Cost will
become unblocked. The Root Bridge cannot have UplinkFast enabled.
UplinkFast is configured globally for all VLANs on the switch:
Switch(config)# spanning-tree uplinkfast
(Reference: http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_35_se/configuration/guide/swstpopt.html)
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