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Catalyst 3500 GigaStack Duplex mode

I have three Cat3500XL switches and four gigastack modules available. I am currently using only three with one cable between each switch. The default mode on these ports seems to be half duplex and the ports will settle into that mode. I have tried reconfiguring the speed to 1000 and duplex to full but they won't settle into that mode. Port negotiation never stops. The second issue I cannot resolve is redundancy in a loop configuration (Connecting SW1 to SW2 and SW2 to SW3 and then SW3 back to SW1). Spantree just won't resolve it. CatOS is 12.0(5.3)WC(1)

Cisco Employee

Re: Catalyst 3500 GigaStack Duplex mode


Sounds like you may be hitting either of these bugs:

CSCdt76136 GBIC reports as LX and will not go half duplex

3548-XL's using CX Gigastack GBIC's may see them as LX GBIC's. This may or may not cause them to not be able to do half duplex.

Workaround: Issue a [no shut] command on the interface prior to installing the gigastack

or copper GBIC’s.


This bug is a duplicate of CSCdr22471 - Razor Gigport0/1 is showing many errors after reload and no-shutdown

CSCdr22471 Release-note:

Gigabit port with Gigastack GBIC, experiences errors in following two cases.

Case 1:

- The port is administratively shutdown (disabled)

- The system is restarted, and then port is enabled.

Case 2:

- The port is administratively shutdown (disabled)

- The Gigastack GBIC is removed and inserted back.

- The port is enabled.

The Workaround for this issue is to remove the GBIC and insert again after enabling the port.

CSCds84479 Link Flap When force Duplex of Razor to Full-Duplex:

When you connect two switches by GigaStack GBICs and you manually set duplex mode

to full duplex or to autonegotiate on both ends, the link sometimes does not stabilize.

The workaround is to remove and reinsert one of the GBICs.

Comment …

Be aware of this caveat when Gigastack GBICs are involved in any config activity.

Always use auto-negotiation with Gigastack GBICs. There will be no sw fix for this.

Hope it helps


New Member

Re: Catalyst 3500 GigaStack Duplex mode

Are you saying that Gigastack GBICs will run FULL Duplex?

New Member

Re: Catalyst 3500 GigaStack Duplex mode

Yes GBICs do run full duplex. I am running a 1 gig fibre link between buildings (Multimode Fibre). This provides the link to the other 3548. (48 port switches with 2 gbics). So far no problems. I check the stats on the switches every week so far no problems and no real errors.

New Member

Re: Catalyst 3500 GigaStack Duplex mode

The gigastack modules I am using are two port copper modules used to stack the switches not the fiber or 1000BaseT modules. They use a propriatary link cable that looks like RJ45. The port can be reconfigured by turning off autonegotiate and setting the speed and duplex. They go green but then start flashing orange within seconds.

New Member

Re: Catalyst 3500 GigaStack Duplex mode

Drew, in my experience (I have 3524's and 3548's with GigaStack GBICs), they will only run full duplex if there is only one cable attached to it. If both ports on the GBIC are used, it runs half, and the other end of the link runs half. So, if you want to run full duplex, use one port on it only, leave the other port empty. Hope that helps!

New Member

Re: Catalyst 3500 GigaStack Duplex mode


Some people before already told you that full duplex only will work with just 1 cable connected between 2 switches. If you connect more switches it wil be seen as a bus and opperate in half duplex mode.

For the oher issue the loop back for redundancy, we are using it with succes. We are running 12.0(5.2)XU.

For futher questions on this item you may contact me on

New Member

Re: Catalyst 3500 GigaStack Duplex mode

I think I can remember something with the Gbic modules on a 3500 switch.

If you use 2 switches to form 1 stack you can run 1gb full on this link, however if you stack more than 3 switches the fastest it will go is 1gb half duplex !

I'm not sure if this issue has been resolved in the latest ios version ?



New Member

Re: Catalyst 3500 GigaStack Duplex mode

Check out

You will find in there the following:

Point-to-Point Connections

A point-to-point connection operates in full-duplex mode. Figure 2-7 shows an example of supporting switches with point-to-point connections

Cascaded Stack Connections

You can connect from three to nine switches in a cascaded stack configuration. The cascaded stack operates in half-duplex mode. Figure 2-8 shows the connections on some supporting switches.

So, in the configuration you're using, it's cascaded and will not work in full duplex mode.

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