speed/duplex mismatches

[hmc2500]

I know that setting one side of a port to auto/auto and the other side to 100/full will cause problems.

however can 1000/full and one side and auto/auto on the other side (both ports 1gbps capable) cause problems? 

and what about 1000/auto and 1000/full. 

does anyone know? It's hard to tell sometimes if this is causing issues. Looking at the runts and drops is not always helpful. And sometimes the port light has issues but does not go amber.

[Richard Burts · Accepted solution]

In general the issue is that if you hard code speed on one peer and have the other peer try to negotiate then negotiation fails. And if negotiation for speed is not successful then the peers do not negotiate duplex. And if duplex is not negotiated then the default is half duplex. So If you hard code speed on one peer you are likely to get duplex mismatches.

And a similar kind of issue is if you let speed negotiate and hard code duplex on one peer then duplex is not negotiated and the peer who attempted to negotiate will choose half duplex, which results in duplex mismatch. 

So the general rule is that if one peer is hard coded then both peers should be hard coded.

[Joseph W. Doherty]

I believe (?) that auto speed should generally always be able to determine the speed even if the other side had a hard coded speed. However, duplex is a different story. Generally a hard coded "full" requires the other side to also be hard coded as "full" as if one side is coded "auto" and the other side is not "auto", the auto side will run as "half".

Where it might get further confusing, on copper Ethernet, all gig capable switches normally assume auto will be the default and half duplex never used. On some modern switches, you might also fine half duplex cannot be configured at all. (For example, a couple of years ago, we installed some modern non-Cisco switches and found we were unable to configure triple speed copper Ethernet ports as 10/half, which some of our old/special hosts still required.)

[Richard Burts]

I am glad that our explanations have been helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.