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12-5
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 12 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure 12-3 UplinkFast Example Before Direct Link Failure
If Switch C detects a link failure on the currently active link L2 on the root port (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 Figure 12-4. This change takes
approximately 1 to 5 seconds.
Figure 12-4 UplinkFast Example After Direct Link Failure
Understanding BackboneFast
BackboneFast detects indirect failures in the core of the backbone. BackboneFast is a complementary
technology to the UplinkFast feature, which responds to failures on links directly connected to access
switches. BackboneFast optimizes the maximum-age timer, which determines the amount of time the
switch stores protocol information received on an interface. When a switch receives an inferior BPDU
from the designated port of another switch, the BPDU is a signal that the other switch might have lost
its path to the root, and BackboneFast tries to find an alternate path to the root.
BackboneFast, which is enabled by using the spanning-tree backbonefast global configuration
command, starts when a root port or blocked port on a switch receives inferior BPDUs from its
designated switch. An inferior BPDU identifies a switch that declares itself as both the root bridge and
the designated switch. When a switch receives an inferior BPDU, it means that a link to which the switch
is not directly connected (an indirect link) has failed (that is, the designated bridge has lost its connection
to the root switch). Under spanning-tree rules, the switch ignores inferior BPDUs for the configured
maximum aging time specified by the vlan vlan-id global configuration command.
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
Blocked port
87817
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
UplinkFast transitions port
directly to forwarding state.
Link failure
87818