Redundant Links with STP
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When a link fault occurs on the primary root link, UplinkFast transitions
the blocked port to a forwarding state. UplinkFast changes the port without
passing through the listening and learning phases, which allows the switch to
skip the normal convergence time and start forwarding in about 3 to 4 sec-
onds instead of the normal 50 seconds.
Cisco has designed UplinkFast to work with its access layer switches, not
its core switches, because the switch running UplinkFast must not be the root
bridge.
UplinkFast Configuration
When configuring UplinkFast, remember that all VLANs on the switch are
affected and that you cannot configure UplinkFast on individual VLANs.
To configure UplinkFast on a set-based switch, use the set spantree
uplinkfast
command:
Todd5000> (enable) set spantree uplinkfast ?
Usage: set spantree uplinkfast <enable> [rate <station_update_rate>]
[all-protocols <off|on>]
set spantree uplinkfast <disable>
The options are really just enable or disable. However, the station update
rate value is the number of multicast packets transmitted per 100 millisec-
onds (by default, it is set to 15 packets per millisecond). It is not recom-
mended that you change this value.
The switch will provide you with an output describing what the command
changed on the switch, as shown here:
Todd5000> (enable) set spantree uplinkfast enable
VLANs 1-1005 bridge priority set to 49152.
The port cost and portvlancost of all ports set to above 3000.
Station update rate set to 15 packets/100ms.
uplinkfast all-protocols field set to off.
uplinkfast enabled for bridge.
Todd5000> (enable)
The VLAN priorities are automatically changed to 49152 and the port costs
are set to above 3000. These are changed to make it unlikely that the switch
will become the root switch.
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