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GBIC loop configurations not supported before IOS software Release 12.0(5)XU
GigaStack Gigabit Interface Converters (GBICs) installed in a stack support a loop configuration only if every device in the stack is running IOS software Release 12.0(5)XU (April 2000) or higher.
For earlier software releases, an illegal loop is created under the following conditions:
•A GigaStack GBIC port within a daisy-chain stack connection is connected to a GigaStack GBIC port of another device in the stack that already has a GigaStack GBIC port connected to a device in that stack; both ports are in the same stack. See .
Figure 1 Incorrect Cabling of Switches in a Stack
Figure 2 Incorrect Cabling in a Single Switch
Prior to Release 12.0(5)XU, loop causes excessive collision errors on the port and might cause the link to become unstable. This instability decreases performance on the links, and communication between the switches in the stack is adversely affected. For correct configuration scenarios, see the Catalyst GigaStack Gigabit Interface Converter Hardware Installation Guide.