Duplex mismatch confusion

[steven.crutchley]

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?

[smitesh kharecha]

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

[flokki123]

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

[Peter Paluch · Accepted solution]

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

[flokki123]

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

[Peter Paluch]

Hi Florian,

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

Best regards,

Peter

[steven.crutchley]

Not a problem. Thank you for the help.