Understanding How Backbonefast Works - Cisco WS-C4003 - Catalyst 4000 Chassis Switch Software Configuration Manual

Chapter 8 Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast, and Loop Guard
To disable UplinkFast on a switch, perform this task in privileged mode:
Task
Step 1
(Optional) Disable UplinkFast processing on the
switch and restore the default bridge priority, port
cost, and port-VLAN cost values.
Step 2 (Optional) Disable UplinkFast processing on the
switch without affecting the bridge priority, port
cost, and port-VLAN cost values.
Step 3
Verify that UplinkFast is enabled.
This example shows how to disable UplinkFast on the switch and restore the default bridge priority, port
cost, and port-VLAN cost values:
Console> (enable) clear spantree uplinkfast
This command will cause all portcosts, portvlancosts, and the
bridge priority on all vlans to be set to default.
Do you want to continue (y/n) [n]? y
VLANs 1-1005 bridge priority set to 32768.
The port cost of all bridge ports set to default value.
The portvlancost of all bridge ports set to default value.
uplinkfast all-protocols field set to off.
uplinkfast disabled for bridge.
Console> (enable) show spantree uplinkfast
uplinkfast disabled for bridge.
Console> (enable)

Understanding How BackboneFast Works

BackboneFast provides fast convergence in the network backbone after a spanning tree topology change
occurs. A switch detects an indirect link failure (the failure of a link to which the switch is not directly
connected) when the switch receives inferior BPDUs from its designated bridge on its root port or
blocked ports. These inferior BPDUs indicate that the designated bridge has lost its connection to the
root bridge. An inferior BPDU identifies a single switch as both the root bridge and the designated
bridge. Under normal spanning tree rules, the switch ignores inferior BPDUs for the configured
maximum aging time (specified by 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. 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.
If the switch has alternate paths to the root bridge, it transmits Root Link Query PDUs out all alternate
paths to the root bridge. If the switch determines that it still has an alternate path to the root, it causes
the maximum aging time on the ports on which it received the inferior BPDU to expire. If all the alternate
paths to the root bridge indicate that the switch has lost connectivity to the root bridge, the switch causes
the maximum aging times on the ports on which it received an inferior BPDU to expire. If one or more
alternate paths can still connect to the root bridge, the switch makes all ports on which it received an
inferior BPDU its designated ports and moves them out of the blocking state (if they were in blocking
state), through the listening and learning states, and into the forwarding state.
78-12647-02
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
Understanding How BackboneFast Works
Command
clear spantree uplinkfast
set spantree uplinkfast disable
show spantree uplinkfast
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