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16-7
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-10
Chapter 16 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
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.
If two switches in the stack have alternate paths to the root, only one of the switches performs the
fast 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 switch 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.
• A switch other than the stack root is powered off or failed.
• A link fails between stack ports on the multidrop backbone.
Limitations
These limitations apply to CSUF:
• CSUF uses the GigaStack GBIC module and runs on all Catalyst 3550 switches, all Catalyst 3500
XL switches, Catalyst 2950 switches with GBIC module slots, and only on modular Catalyst 2900
XL switches that have the 1000BASE-X module installed.
• Up to nine stack switches can be connected through their stack ports to the multidrop backbone.
Only one stack port per switch is supported.
• Each stack switch can be connected to the spanning-tree backbone through one uplink.
• If the stack consists of a mixture of Catalyst 3550, Catalyst 3500 XL, Catalyst 2950, and
Catalyst 2900 XL switches, up to 64 VLANs with spanning tree enabled are supported. If the stack
consists of only Catalyst 3550 switches, up to 128 VLANs with spanning tree enabled are
supported.