Understanding How Backbonefast Works - Cisco WS-C6506 Software Manual

Understanding How BackboneFast Works

Figure 9-1
Switch A
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 without going through
the listening and learning states, as shown in
5 seconds.
Figure 9-2
Switch A
Understanding How BackboneFast Works
BackboneFast is initiated when a designated port on a network device receives inferior BPDUs from its
designated bridge. Inferior BPDUs can occur when the designated device loses the root and advertises a
root with a higher bridge ID or the device path/cost to the root is higher than the network device. When
a switch receives an inferior BPDU, it indicates 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 bridge).
Under normal spanning-tree rules, the switch ignores inferior BPDUs for the configured maximum aging
time, as specified by the agingtime variable of the set spantree maxage command.
The switch tries to determine if it has an alternate path to the root bridge. If the inferior BPDU arrives
on a blocked port, the root port and other blocked ports on the switch become alternate paths to the root
bridge. (Self-looped ports are not considered alternate paths to the root bridge.) If the inferior BPDU
arrives on the root port, all blocked ports become alternate paths to the root bridge. If the inferior BPDU
arrives on the root port and there are no blocked ports, the switch assumes that it has lost connectivity
to the root bridge, causes the maximum aging time on the root to expire, and becomes the root switch
according to normal spanning-tree rules.
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
9-4
UplinkFast Example Before Direct Link Failure
(Root)
L1
L2
Switch C
UplinkFast Example After Direct Link Failure
(Root)
L1
L2
Link failure
Switch C
Chapter 9 Configuring Spanning-Tree PortFast, UplinkFast, BackboneFast, and Loop Guard
Switch B
L3
Blocked port
Figure 9-2. This switchover takes approximately 1 to
Switch B
L3
UplinkFast transitions port
directly to forwarding state
OL-8978-04