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Understanding How UplinkFast Works
Understanding How UplinkFast Works
UplinkFast provides fast convergence in the network access layer after a spanning tree topology change
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 (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 with 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 one to five seconds.
Figure 8-2
Switch A
(Root)
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
8-8
Chapter 8
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
UplinkFast Example After Direct Link Failure
L1
L2
Link failure
Switch C
Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast, and Loop Guard
Switch B
L3
Switch B
L3
UplinkFast transitions port
directly to forwarding state
Figure
8-2). This switchover
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