duplex settings and speed change cause link down

[Steven24]

I have a live case, change duplex and speed on CE router from 100 to 10 WAN interface, link goes down. I would like to know why and detials of duplex and settings mechanisam. thanks!

[Reza Sharifi]

If the link goes down and does not come back up, that is probably because the speed does not match the other side of the connection. So, you have speed and duplex mismatch.

HTH

[Giuseppe Larosa]

Hello Steven24,

the short answer is that speed is detected with Link Pulses that are not frames but short signals on the wire.

The duplex settings can be negotiated using autonegotiation described for example here

https://en.wikipedia.org/wiki/Autonegotiation

To be noted in autonegotiation the two sides can signal both capabilities of speed and duplex

>> Each device declares its technology abilities, that is, its possible modes of operation, and the best mode is chosen from those shared between them, with higher speed preferred over lower, and full duplex preferred over half duplex at the same speed. 

As noted by Reza if you hardcode the speed and duplex and the other side has different speed settings the link will go down.

It is better to rely on autonegotiation instead of using hard coded speed and duplex.

These commands should be used only if one of the two devices does not support autonegotiation.

Hope to help

Giuseppe

[Joseph W. Doherty · Accepted solution]

Speed on both sides must match. If the interfaces support multi-speed, and if they are set to auto negotiation they should agree on the same speed and the link will come up. (If you hard code speed, you can run into issues keeping both sides from having the same speed setting. Again, if it doesn't match it won't work.)

Duplex should (!!!) match. Unlike speed, sometimes a link will "work" with a duplex mismatch, but if it does, it tends to pass traffic very, very slowly. Again, if both sides are set to auto, they should agree on the duplex (usually full, when dealing with switches). If one side is hard coded to full, and the other is auto, the auto will run in half.

Auto, I recall (?) besides working out speed and duplex also has some additional error detection (also on some Cisco switches, auto also support auto MDI/MDI-X). This is another reason why auto is the preferred port setting (as generally recommended by all hardware vendors). Generally anything built during this century should work with auto but one really old equipment it might be "flakey".)

Oh, in theory a hard coded interface will come on-line a bit faster than an auto configured interface, so some engineers (old school) will suggest using hard coded settings for infrastructure links. However, I would still recommend using auto on all ports that support it unless you have one that only works with hard coded settings.