
8-3
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard
Understanding How UplinkFast Works
Understanding How UplinkFast Works
UplinkFast provides fast convergence using uplink groups in the network access layer after a spanning
tree topology change. 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 (not including self-looped ports). The uplink group provides an alternate path in case the
currently forwarding link fails.
Note
UplinkFast is most useful in wiring-closet switches that have a limited number of active VLANs. This
enhancement might not be useful for other types of applications and should not be enabled on backbone
or distribution layer switches.
Figure 8-1
shows an example UplinkFast network topology. Switch A, the root switch, is connected
directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that is connected
to Switch B over link L3 is in blocking state.
Figure 8-1
UplinkFast Example Before Direct Link Failure
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 immediately, without
transitioning the port through the listening and learning states (as shown in
Figure 8-2
). This switchover
takes approximately 1 to 5 seconds.
Figure 8-2
Example of UplinkFast After Direct Link Failure
L1
L2
L3
Switch C
Switch A
(Root)
Switch B
Blocked port
1
1241
L1
L2
L3
Switch C
Switch A
(Root)
Switch B
UplinkFast transitions port
directly to forwarding state
Link failure
1
1242