Speed and duplex strange issue

[ShoaibM]

Hi all,

I need to understand the speed and duplex settings of the port. I am trying to hard code the speed and duplex settings of the interface which apparently seems a simple procedure. So far what i have learned is you can hard code the interface speed and duplex settings. But in my lab i have the following situation.

                             Status "notconnect"
SW-3560 <-----------------------------> SW-2960
speed 100                                                         speed 100
duplex full                                                          duplex full

                             Status "notconnect"
SW-3560 <-----------------------------> SW-2960
speed 100                                                         speed 100
duplex auto                                                       duplex auto

                             Status "notconnect"
SW-3560 <-----------------------------> SW-2960
speed auto                                                        speed auto
duplex full                                                         duplex full

                             Status "connected"
SW-3560 <-----------------------------> SW-2960
speed 100                                                         speed auto
duplex full                                                          duplex auto

****** no "DUPLEX MISMATCH DISCOVERED" log messages ******

                             Status "connected"
SW-3560 <-----------------------------> SW-2960
speed auto                                                        speed 100
duplex auto                                                       duplex full

******* "DUPLEX MISMATCH DISCOVERED" log messages on both switches ******

**** if i run "sh controllers ethernet-controller fa 0/4" command on both switches. I can see collisions on 3560 interface in the "TRANSMIT" column and similarly on 2960 in the "RECIEVE" column.

Ports on both these switches are 10/100 capable.

I have tried to go through books, cisco online documents and previous community threads but so far i could not find anything which can explain this behaviour of the interface.

Any idea why is this?

Many thanks.

[Joseph W. Doherty · Accepted solution]

"I am using cat 6 straight cable. I will check with crossover cable and let you know.
Both switches support MDI-X feature. I think you are correct."

Leo is likely correct. "Like kind" of Ethernet ports, connected back-to-back, need a cross over cable unless the ports support auto mdi/mdi-x (which as Leo noted, is disabled when you turn off "auto").

Also as Leo noted, hard coding speed/duplex is very old school, like last century. Generally, network vendors, this century, recommend using auto/auto. (About the only advantage of using hard code speed duplex, assuming auto/auto works, a link will come "up" a bit faster hard coded, but then you give up some built in error checking.)

[ShoaibM]

Thank you for your response Joseph.

Yes, Leo is indeed correct. I didn't think about MDI-X. This is interesting to know about auto/auto setting. Thank you for sharing your experience.

[Joseph W. Doherty · Accepted solution]

BTW, auto/auto doesn't always imply there's auto MDI/MDI-X too - i.e. it's platform dependent.

[ShoaibM]

Thank you, Joseph. I understand now. I will come back once I check with crossover cable just for my understanding sake.

[ShoaibM]

Hi @Leo Laohoo and @Joseph W. Doherty
You were right. With the crossover cable, there is no issue when I change the speed/duplex of the interface. Thank you for your help.