Collisions on WS-C3524-XL-EN

Answered Question
Jun 10th, 2009

Hello everyone,

I have a number of switches on the network that are linked on the G0/1 port with a CX_GIGASTACK module.

I have numerous stacks throughout the network.

Some appear to be connecting at the following...

Auto-duplex (Full), 1000Mb/s, media type is CX_GIGASTACK (0 collisions)

And some connect at

Auto-duplex (Half), 1000Mb/s, media type is CX_GIGASTACK (56 collisions)

All switches are running the same OS Version 12.0(5)WC17

1.) Is there something configured wrong with the half duplex interfaces? I thought that is what these GigaStack modules connected at.

2.) They seem to be getting the collisions. In the switch above I cleared the counters and it started up again with the collisions (56) and just keeps climbing. Not at a fast rate but still goes up.

The stacks that connect at Auto-duplex (Full), 1000Mb/s do not seem to have any collisions.

This doesn't appear to be affecting the network in any way its just bugging me.

Thank you for your help.

Matt

I have this problem too.
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Correct Answer by Leo Laohoo about 7 years 6 months ago

Thanks for the rating.

Correct Answer by Giuseppe Larosa about 7 years 6 months ago

Hello Matt,

>> The stacks that connect at Auto-duplex (Full), 1000Mb/s do not seem to have any collisions.

this is correct full duplex links should never see collisions

Conversely half-duplex links experience collisions this is normal: a collision causes a retrasmission at ethernet level.

if the collision rate is low this can be tolerated.

Be aware that performances are reduced to roughly 330 Mbps per direction.

About the ports negotiating 1000 half with normal ports you could try to force another negotiation with a shut/no shut cycle but I'm afraid here this is out of discussion.

Hope to help

Giuseppe

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sachinraja Wed, 06/10/2009 - 09:19

Hello Matt

collisions can be visible only on half-duplex links.. with Full duplex / 1000 Mbps, you will not notice collisions... There are certain combinations of duplex/speed settings which could cause interfaces to auto-negotiate to half-duplex.. Best practice reco is to leave auto,auto both the sides (for trunks) and to hard code speed/duplex for server ports.. Attaching a document which clearly describes the speed/duplex combinations..

Hope this helps.. all the best.. rate replies if found useful..

Raj

Correct Answer
Giuseppe Larosa Wed, 06/10/2009 - 09:23

Hello Matt,

>> The stacks that connect at Auto-duplex (Full), 1000Mb/s do not seem to have any collisions.

this is correct full duplex links should never see collisions

Conversely half-duplex links experience collisions this is normal: a collision causes a retrasmission at ethernet level.

if the collision rate is low this can be tolerated.

Be aware that performances are reduced to roughly 330 Mbps per direction.

About the ports negotiating 1000 half with normal ports you could try to force another negotiation with a shut/no shut cycle but I'm afraid here this is out of discussion.

Hope to help

Giuseppe

glen.grant Wed, 06/10/2009 - 14:24

What you are seeing is normal and the switch auto negotiates the speed and duplex depending on the stack configuration and how the cables are run between the devices in the stack. . You cannot force the speed and duplex in a gigastack arrangement the software determines how the gbic will negotiate the link. Half duplex is the normal state in what they call a cascaded cable connection style , no way around this .

Here is more info explaining this.

http://www.cisco.com/en/US/products/hw/switches/ps607/products_tech_note09186a00800a2cac.shtml

Leo Laohoo Wed, 06/10/2009 - 14:45

It doesn't matter what IOS you are running. If you are using GigaStacking GBIC to more than 2 switches, the ports will auto-negotiate to HALF duplex.

The only time you get FULL duplex is when only two GigaStacking GBIC are connected to each other using only ONE cable.

Does this help?

Leo Laohoo Fri, 06/12/2009 - 18:35

Hi Matt,

Just want to let you know that the WS-C3500-XL-EN, WS-C2950G, and WS-C3550 series switches are now end-of-sale. The GigaStacking GBIC is also end-of-sale.

The options for upgrade will be the following:

WS-C2975 Layer 2 Switch Only:

Cisco Catalyst 2975 Switch with LAN Base Software Data Sheet

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps10081/data_sheet_c78-502660.html

WS-C3750 Layer 2/3 Switch:

Cisco Catalyst 3750 Data Sheet

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps10081/data_sheet_c78-502660.html

Cisco Catalyst 3750-E Series Switches Data Sheet

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps7077/product_data_sheet0900aecd805bbe67.html

The following switches has the new StackWise/StackWise Plus feature that will allow you to stack your switches without the "cluster" feature.

Hope this helps.

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