Understanding Backbonefast - Cisco IE 3000 Software Configuration Manual

Chapter 18 Configuring Optional Spanning-Tree Features
Figure 18-3
Switch A
(Root)
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 interface on Switch C and transitions it to the forwarding state without
going through the listening and learning states, as shown in
approximately 1 to 5 seconds.
Figure 18-4
Switch A
(Root)

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 controls 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 interface 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 switch 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 spanning-tree vlan vlan-id max-age global configuration
command.
OL-13018-01
UplinkFast Example Before Direct Link Failure
L1
L2
Blocked port
Switch C
UplinkFast Example After Direct Link Failure
L1
L2
Link failure
Switch C
Understanding Optional Spanning-Tree Features
Switch B
L3
Figure
Switch B
L3
UplinkFast transitions port
directly to forwarding state.
Cisco IE 3000 Switch Software Configuration Guide
18-4. This change takes
18-5