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GigaStack GBIC link flaps and never stabilizes due to misconfiguration of duplex or link negotiation


Core issue

Configuring a GigaStack port's duplex or negotiation to a setting other than auto might cause the link to flap and never stabilize.


When two switches are connected by a GigaStack Gigabit Interface Converter (GBIC) and duplex is forced to full on both ends, the link may flap and never stabilize.

When reconfigured to auto duplex on both ends, the link still does not stabilize.

After reinserting one of the razors, the link finally stabilizes.

This is also seen when link negotiation is shut off. The duplex must always be auto-negotiated when a GigaStack GBIC is inserted.

To resolve this issue, perform these steps:

  1. Configure the GigaStack ports to auto-negotiate duplex by issuing the duplex auto interface command.
  2. Configure the GigaStack port to negotiate the link by issuing the negotiation auto interface command.
  3. Once complete, remove and reinsert the GigaStack GBIC.
  4. The show interfaces command shows the negotiated state for the GigaStack.

    Check for these values in the show interfaces command output:

    Auto-duplex (Half), 1000Mb/s, media type is CX_GIGASTACK

Note: Link and duplex auto-negotiation are the default configuration for the GigaStack interfaces.


For more information regarding GigaStack configuration, refer to Catalyst Switch GigaStack Configuration and Implications.

Device connected to switch

Another Switch