Cisco Catalyst 3750-E

20-7
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-08
Chapter 20 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Each switch in the stack decides if the sending switch is a better choice than itself to be the stack root
of this spanning-tree instance by comparing the root, cost, and bridge ID. If the sending switch is the
best choice as the stack root, each switch in the stack returns an acknowledgement; otherwise, it sends
a fast-transition request. The sending switch then has not received acknowledgements from all stack
switches.
When acknowledgements are received from all stack switches, the Fast Uplink Transition Protocol on
th
e sending switch immediately transitions its alternate stack-root port to the forwarding state. If
acknowledgements from all stack switches are not obtained by the sending switch, the normal
spanning-tree transitions (blocking, listening, learning, and forwarding) take place, and the
spanning-tree topology converges at its normal rate (2 * forward-delay time + max-age time).
The Fast Uplink Transition Protocol is implemented on a per-VLAN basis and affects only one
s
panning-tree instance at a time.
Events that Cause Fast Convergence
Depending on the network event or failure, the CSUF fast convergence might or might not occur.
Fast convergence (less than 1 second under normal network conditions) occurs under these
c
ircumstances:
• The stack-root port link fails.
If two switches in the stack have alternate paths to the root, only one of the switches performs the
f
ast transition.
• The failed link, which connects the stack root to the spanning-tree root, recovers.
• A network reconfiguration causes a new stack-root switch to be selected.
• A network reconfiguration causes a new port on the current stack-root switch to be chosen as the
stack-root port.
Note The fast transition might not occur if multiple events occur simultaneously. For example, if a stack
member is powered off, and at the same time, the link connecting the stack root to the spanning-tree root
comes back up, the normal spanning-tree convergence occurs.
Normal spanning-tree convergence (30 to 40 seconds) occurs under these conditions:
• The stack-root switch is powered off, or the software failed.
• The stack-root switch, which was powered off or failed, is powered on.
• A new switch, which might become the stack root, is added to the stack.
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.