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New Member

Duplex mismatch confusion

I have the current setup...

(warning... the hostnames and interface numbers in this explanation have been changed for simplicity puroposes )

Switch A is connected Switch B via interface fa0/1.

Switch B is connected to Switch A via interface fa0/1 also.

Both ports are 10/100 Ethernet Ports capable of full duplex.

Switch A is a L3 Cisco WS-3750-48-TS-S

Switch B is a L2 Cisco WS-C2950-24

Switch A  interface fa0/1 is set to:

speed auto

duplex auto

Switch B  interface fa0/1 is also set to:

speed auto

duplex auto

Naturally they negotiate full duplex 100Mb/s

When I go to Switch A and set the fa0/1 duplex to full and the speed to 100 I expect the following to happen...

Switch B can still detect the link speed as 100Mb/s and thus the speed on fa0/1 (for switch B) stays at 100Mb/s (or a-100)

Switch B CANNOT detect the duplex because Switch A is not responding. It thus defaults to half duplex.

voila... a duplex mismatch (I am doing this for the purposes of study. I'm not deliberately making a duplex mismatch on a live environment )

BUT. That is not what happens...

The link simply goes down.

Whats worse, even if I set fa0/1 on switch B to speed 100, duplex full, the link does not come up.

I have to set both ends back to auto speed and auto duplex for everything to come back up.

However if I do things the other way around (leave A as auto speed/duplex and set B to 100/full) the behaviour work perfectly,

I think this issue is with the switch types.

Can anyone help?

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: Duplex mismatch confusion

Florian,

Are you using a cross-over cable? On devices supporting auto-MDIX, setting the duplex and speed manually will deactivate the auto-MDIX function. If you are using a straight-through cable between two switches, this can result in the link going down. This seems to be the case when you define the 100/Full mode statically on your 3750. Note that the 2950 does not support auto-MDIX and can not adapt to the cable type. For these epxeriments, you should be using a cross-over cable to interconnect your switches.

Regarding the speed autosense, with both speed and duplex, the autonegotiation may be disabled (different hardware platforms do this differently - some deactivate the autonegotiation altogether, other leave it running but advertise only the particular static setting - it can get very, very confusing), in which case the FLPs would not be sent, either. However, each speed on Ethernet uses a distinctively different link coding scheme. The opposite device may be able to determine the speed simply at looking on the link coding scheme and voltage levels.

Best regards,

Peter

6 REPLIES

Duplex mismatch confusion

Hi Steven,

In theory it should be exactly the way you have mentioned.

Unfortunately neogotiation sometime fails, in which case Cisco defaults to 10/half settings.

What I propose is, set both interface to auto and then check output of command sh int fa0/1 on both routers.

This will give you the answer, whether it is sync at 10/half or 100/full.

That why, it is always advisable to hard code speed and duplex settings and not to leave them to auto.

Regards,

Smitesh

New Member

Duplex mismatch confusion

hi all,

i was just wondering how switch B is still able to sense the speed of the connetion.

cause i know that switch B will send a FLP to switch A to check for the settings. as switch A is hard coded it doesnt respond to the FLP and thus switch B will set itself to half duplex, as ethernet is by default a half duplex technology.

but what about the speed, how is it possible to get the speed for switch B.

i thought the speed is also checked via the FLP.

regrads,

florian

Cisco Employee

Re: Duplex mismatch confusion

Florian,

Are you using a cross-over cable? On devices supporting auto-MDIX, setting the duplex and speed manually will deactivate the auto-MDIX function. If you are using a straight-through cable between two switches, this can result in the link going down. This seems to be the case when you define the 100/Full mode statically on your 3750. Note that the 2950 does not support auto-MDIX and can not adapt to the cable type. For these epxeriments, you should be using a cross-over cable to interconnect your switches.

Regarding the speed autosense, with both speed and duplex, the autonegotiation may be disabled (different hardware platforms do this differently - some deactivate the autonegotiation altogether, other leave it running but advertise only the particular static setting - it can get very, very confusing), in which case the FLPs would not be sent, either. However, each speed on Ethernet uses a distinctively different link coding scheme. The opposite device may be able to determine the speed simply at looking on the link coding scheme and voltage levels.

Best regards,

Peter

New Member

Re: Duplex mismatch confusion

hi peter,

it was actually stevens post, i just had a additional question regarding the speed sense, but thanks for your answer.

so if a switch, set to auto/auto, could still sense the speed even though it doesnt get a FLP through the link coding scheme or by looking at the voltage levels running on the medium.

florian

Cisco Employee

Re: Duplex mismatch confusion

Hi Florian,

Aaah, yes, I was kind of too hasty to answer. Thank you for correcting me. Steven, I apologize

Best regards,

Peter

New Member

Duplex mismatch confusion

Not a problem. Thank you for the help.

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