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Chapter 5
Using Spanning Tree with VLANs
Warning: Spantree port fast start should only be enabled on ports
connected to a single host. Connecting hubs,
concentrators, switches, bridges, etc. to a fast start
port can cause temporary spanning tree loops. Use with
caution.
Spantree ports 2/1-12 fast start enabled.
Todd5000> (enable)
Notice the nice warning received on the switch console when PortFast was
turned on. Also notice that we were able to turn on all 12 ports of our
10/100 card.
To configure PortFast on a 1900 switch, use the spantree start-
forwarding
command:
1900A#config t
1900A(config)#int e0/1
1900A(config-if)#spantree start-forwarding
1900A(config-if)#
This must be configured on each port you want to run PortFast.
UplinkFast
UplinkFast is used to minimize network downtime by ensuring that network
loops do not occur when the network topology changes. The problem is that
STP convergence time is very time-consuming, so network loops can occur
when the convergence is taking place. UplinkFast can reduce the conver-
gence time during a link failure or a topology change.
Another problem with STP and the convergence time is that some hosts
will not be available for communication during the convergence time
because STP has disabled ports on a switch during convergence. The key is
decreased convergence time, which UplinkFast was developed to provide.
UplinkFast allows a blocked port on a switch to begin forwarding frames
immediately when a link failure is detected. For the switch to change a port
from blocking to forwarding mode, UplinkFast must have direct knowledge
of the link failure.
In order to utilize UplinkFast, several criteria must be met. First, Uplink-
Fast must be enabled on the switch. The switch must have at least one
blocked port, and the failure must be on the root port. If the failure is not on
a root port, UplinkFast will ignore it and normal STP functions will occur.
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