Article ID: 4674
Protocols/Features
Auto-negotiation
Goals
Results of various Speed and Duplex settings
Symptoms
Collisions
Runts
Fragments
Alignment Errors
CRC errors
FCS errors
OutErrors
Poor performance
Duplex mismatch
Solution
Partnered 10/100/1000 stations may be configured to one of the following settings:
- Auto-negotiate
- 1000 Mb/s, Full Duplex
- 1000 Mb/s, Half Duplex
- 100 Mb/s, Full Duplex
- 100 Mb/s, Half Duplex
- 10 Mb/s, Full Duplex
- 10 Mb/s, Half Duplex
It is best to use identical settings for partnered devices. If this is not done, a reliable connection may or may not be readily established, depending upon the combination of settings used.
If using auto-negotiation on one end and hard-setting on the other, it is important to keep in mind that the auto-negotiated station will, in the absence of a negotiation response from its link partner, revert to Half Duplex and the speed of the link partner.
If hard-setting Speed and Duplexing, you
must use identical settings on both ends, in order to efficiently communicate.
Half Duplex operation at one end with Full Duplex operation at the other, results in excess collisions as detected by the HD station; with consequent retransmissions, and reduced performance. RMON/Statistics will show Collisions, Runts, Fragments, Alignment Errors, and CRC/FCS Errors. You may also see a flashing red LED on the port, indicating the collisions.
Note that Collisions, Runts, and Fragments are all normal byproducts of operation in a straight Half Duplex environment, and require no corrective action.
The valid possibilities for Speed/Duplex settings, assuming that the auto-negotiating station is capable of performing at the hard-set rate of its link partner:
- Auto-negotiate at both ends
- Auto-negotiate at one end, and Hard-set 10HD at the other end
- Auto-negotiate at one end, and Hard-set 100HD at the other end
- Auto-negotiate at one end, and Hard-set 1000HD at the other end (inefficient)
- Hard-set the same Speed/Duplex at both ends
It is generally advised to use auto-negotiation at both ends, for ease of configuration. However, auto-negotiation is implemented with sufficient variation to make it less than 100% compatible among all manufacturers and all product lines. Its use can, in certain cases, result in symptoms indicative of a duplexing mismatch (see above). If these symptoms become evident, one or both sides may be hard-set as a workaround.
Focusing on gigabit ports,
4593 clearly indicates that not all tri-speed devices can be hard-set. One must operate within the applicable constraints of the hardware in question. If your default is to hard-set, then given equipment which will not support that ability, you will need to use auto-negotiation - and this will only become an issue if auto-negotiation then does not work properly in that particular situation.
