Hardcode Duplex on Ports?

[tequilaschramm]

Is it best to hard code all ports to full duplex instead of leaving them at Auto? And if you do hard code these ports to full, will it cause problems if some of your server nics are set to auto?

Thanks!

[Peter Paluch]

Hello Tequila,

It is generally a bad idea to set duplex (and speed) manually if it has been proven that the autonegotiation works correctly. The major risk with manually setting duplex and speed is that on many platforms, this deactivates both auto-MDIX and autonegotiation. This could under circumstances lead to duplex mismatches if the opposite device expects to learn about duplex setting via autonegotiation.

See the following discussion for more information:

https://supportforums.cisco.com/message/3401724#3401724

Best regards,

Peter

[tequilaschramm]

Thanks for your help!

[Jonathancert_2]

I agree with Peter, not a good ideal to hard code.  Was having latency problems with some of the PC's connected to VOIP.  Previous Network admin had hard coded ports to 1GIG.  After changing setting to auto found out there was cabling issues on the CAT6.  Had to re-terminate some of the wall jacks.  Wierd thing is the POE worked, but VOIP would be 100Mbps, sometimes even half duplex.

Jonathan,

[Edwin Summers]

Jonathan and Peter have already put this to rest, but I will add an additional experience for thought.  I recently worked with some equipment in a transportable kit that included a Cisco 2811 and some other devices.  We found out that one of the devices connected to the 2811 would effectively enter a race condition if the entire system was powered on simultaneously (such as initial power on, or recovery from a power loss).  If the device connected to the router enabled its interface first, it would try to negotiate even though it was set to 100 / full-duplex.  (Obviously, without the router interface up to negotiate with, it would default to half-duplex.)  We confirmed with the manufacturer that this was a (previously unknown) bug.

I have not encountered any problems with autonegotiation for several years now.  The last problem I experienced was with some packetshapers about 10-11 years ago.  Conventional wisdom I've heard of late is auto-negotiate.  Of course, that's just one man's experience.

Best,

Ed