Events That Cause Fast Convergence; Backbonefast - Cisco Catalyst 2960-XR Configuration Manual

BackboneFast

Events That Cause Fast Convergence, on page 86

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 circumstances:
• The stack-root port link fails.
• 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
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.
Related Topics
Enabling UplinkFast for Use with Redundant Links, on page 95
UplinkFast, on page 82
Cross-Stack UplinkFast, on page 83
How Cross-Stack UplinkFast Works, on page 84
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 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
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
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If two switches in the stack have alternate paths to the root, only one of the switches performs the fast
transition.
port.
The fast transition might not occur if multiple events occur simultaneously. For example,
Note
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.
Configuring Optional Spanning-Tree Features
OL-29424-01