Recently I came across an issue on a customer location where the connection between the router and the switch had a duplexing mismatch. One side was configured as 100 Full and the other end was Auto/Auto. As a result the Auto/Auto auto configured for 100/Half. Now this location had been running for months without issue, so why all of a sudden did it start having problems? There is no doubt that the configurations had not been changed in a long time.
Firstly the site had multiple routers running GLBP, therefore it is possible that the router in question was not being used, and for what ever reason now some PC were deciding to use this router. Certainly one thing I noticed on this router with the duplex problem is that ‘show glbp brief” was in ‘init’ state for some groups and had ‘unkown’ in some ‘active’ and ‘standby’ columns. There was nothing glaringly obvious with the GLBP config, it was just in an inconsistent state (I didn’t have access to the running config). The logs did not show any significant events which would point to a duplexing error.
What I should also mention the data problem being reported was files copied across the network were being corrupted for a few users.
After more investigation the provider found that their router’s fast Ethernet interface was clocking up errors then, it became obvious that the duplexing was mismatched.
It may be worth doing a standard check on all devices for interfaces with running at 100/half, I do not ever recall having 100mb connection which would run half duplex only ever 100 full. Maybe during the initial progression from 10/half to 100/full there were switches/repeaters which ran 100/half.
So this raises the question; When should I manually set port speed and duplex setting and when should I use Automatic negotiation?
First let’s understand briefly AUTO negotiation of Speed and Duplex.
The negotiation is an OPTIONAL part of IEEE 802.3u Fast Ethernet Standard, although it is fair to say most devices these days would support it, possibly older 100mb devices could be affected.
This standard exchanges link capabilities between devices on the end of the link and determine a common speed and duplex setting, even with this configured on both device, there can still be mismatches due to glitches. (This is the same on Gigabit too – gigabit does not do half duplex)
The best place to use Auto negotiate is where devices come and go, e.g. for desktop or laptop connection which have the potential to be moved etc, and potentially have different capabilities.
With this said it makes perfect sense for fixed/permanent and critical components connection we should set the duplex and speed manually on BOTH devices.
So Router to Switch/ Switch to Switch we should have fixed settings, also remember to add description on ports.
What else is in the mix.
What about IP phone, these are effectively small network switches. It depends on the criticality of your phones, but in general I have seen them as AUTO. And really just treat them like moveable devices e.g. PC
Summary
Use AUTO negotiation (both ends) for movable devices
Use Fixed configuration for (both ends) for critical network components and critical server components.
Key Indicators of Problems
If you see a switch port running at full duplex but see lost of FCS and Alignment errors clocking up there is a good chance the other side is on half duplex.
If you have port running a-100, a-half then it is likey that the a-half is wrong and maybe worth investigating.
References
http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a0080094781.shtml
Required CCO Access:
http://www.cisco.com/en/US/partner/products/hw/switches/ps700/products_tech_note09186a00800a7af0.shtml
