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Disable autonegotiation on switch ports results in interface going down?

Good day all,

I'm labbing some switching configurations as I work on CCENT/CCNA stuff and need some help. I have 2 switches connected on a trunk. SW1 is using port FA0/2 and SW2 is using port FA0/1. I wanted to disable autonegotiation on SW1 to observe the default bahaviors when autonegotiation fails.

On SW1, I set FA0/2 to speed 100 and duplex full. But when I do this, the link goes down. FA0/2 on SW1 is down/down notconnect. Same on FA0/1 on SW2. This is not what I was expecting. My understanding is with autoneg on one side and disabled on the other, speed should still be detected and then duplex should be chosen based on speed (half in this case since speed will be 100Mbps) and the link should be up/up (even though it probably won't work well because of a duplex mismatch).

Can someone shed some light on to why the link is down after turning autoneg off on one side? SW1 is a 2960 and SW2 is a 3550.

Thanks for any help!

7 REPLIES
VIP Super Bronze

Hi,If use auto or full, it

Hi,

If use auto or full, it needs to be configured on both sides of the link:

from Cisco doc:

Both sides of a link should have auto-negotiation on, or both sides should have it off. Cisco recommends to leave auto-negotiation on for those devices compliant with 802.3u.

http://www.cisco.com/c/en/us/support/docs/lan-switching/ethernet/10561-3.html

HTH

Hi, are you using a cross

Hi,

 

are you using a cross cable or a straight one with mdix auto enabled?

 

Best regards,

Milan
 

Super Bronze

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Posting

I would suspect you're using a straight thru cable, and when you disable auto-neg it will disable auto MDI.  Auto MDI is likely a feature of the 2960 but also likely not a feature of the 3550.

If one side's interface is set to auto, and other is hard coded, both side should agree on speed, but auto side will "assume" half duplex.  Hard coded duplex will be whatever you've set it for.  Of course, if duplex doesn't match, you'll get terrible performance.

Hi, yes, I am using a

Hi, yes, I am using a straight thru cable. So disabling autoneg disables auto mdix as well? I did not know that! I will test turning off autoneg on the 3550 instead and leave autoneg on the 2960 and see if I get the expected results. As mentioned, this is just part of a lab so no worries about duplex mismatch I just want to see what happens. Thanks!

Hi,I guess you will not get

Hi,

I guess you will not get the port Up with straight cable when disabling auto negotiation on either port.

If you have some time to play in your lab, it would be a nice excercise to replace the straight cable with a cross one and repeat your tests.

 

Best regards,

Milan

 

 

 

 

Purple

   If you are going to

   If you are going to hardcode ports then you "have" to use a crossover cable.  You are probably using a straight cable and having mdix correct the ports . in order to work the ports "must" set as auto otherwise the switch will take the port down which is normal in cisco gear. the 3550 is not capable of auto mdix so once you hardcode the 2960 side it will take down the port. 

Thank you all for your

Thank you all for your replies. Yes, it appears the straight through cable was the issue. When disabling autoneg on the 2960, that stopped auto MDIX. So, I turned it back on on the 2960 and disabled it on the 3550 by setting speed 100 and duplex full. I still did not get the results I expected though. I would have expected the 2960 to default to half duplex since autoneg was off on the other side of the link and the speed was 100. Yet sh int status still reports the port as being a-full. How can this be? Am I missing something? Thank you for the help!

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