Understanding How Uplinkfast Works - Cisco WS-C2948G-GE-TX Configuration Manual

Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard

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
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
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 immediately, without
transitioning the port through the listening and learning states (as shown in
takes approximately 1 to 5 seconds.
Figure 8-2
Switch A
(Root)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
shows an example UplinkFast network topology. Switch A, the root switch, is connected
UplinkFast Example Before Direct Link Failure
L1
L2
Blocked port
Switch C
Example of UplinkFast After Direct Link Failure
L1
L2
Link failure
Switch C
Switch B
L3
Switch B
L3
UplinkFast transitions port
directly to forwarding state
Understanding How UplinkFast Works
Figure 8-2). This switchover
8-3