Understanding How Uplinkfast Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Chapter 9 Configuring Spanning Tree PortFast, UplinkFast, BackboneFast, and Loop Guard

Understanding How UplinkFast Works

UplinkFast provides fast convergence after a spanning tree topology change and achieves load balancing
between redundant links using uplink groups. An uplink group is a set of ports (per VLAN), only one of
which is forwarding at any given time. Specifically, an uplink group consists of the root port (which is
forwarding) and a set of blocked ports. The blocked ports do not include self-looping ports. The uplink
group provides an alternate path in case the currently forwarding link fails.
UplinkFast is most useful in wiring-closet switches. This feature may not be useful for other types
Note
of applications.
Figure 9-1
directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that is connected
directly to Switch B is in blocking state.
Figure 9-1
Switch A
(Root)
If Switch C detects a link failure on the currently active link L2 (a direct link failure), UplinkFast
unblocks the blocked port on Switch C and transitions it to the forwarding state without going through
the listening and learning states, as shown in
5 seconds.
Figure 9-2
Switch A
(Root)
78-13315-02
shows an example topology with no link failures. Switch A, the root switch, is connected
UplinkFast Example Before Direct Link Failure
L1
L2
Blocked port
Switch C
UplinkFast Example After Direct Link Failure
L1
L2
Link failure
Switch C
Switch B
L3
Figure 9-2. This switchover takes approximately 1 to
Switch B
L3
UplinkFast transitions port
directly to forwarding state
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4

Understanding How UplinkFast Works

9-3