by Terry Slattery
Auto-negotiate Duplex or not?
Terry Slattery, CCIE #1026, is a senior network engineer with decades of experience in the internetworking industry. Prior to joining Chesapeake NetCraftsmen as a full time consultant, Terry was the founder and CTO of Netcordia, and inventor of NetMRI, a suite of network management products. Terry is the former President and founder of Chesapeake Computer Consultants, Inc., a networking and computer systems training and consulting company. He co-invented and patented the vLab(tm) internet-based remote lab system. He is co-author of the McGraw Hill text Advanced IP Routing in Cisco Networks. Terry led the team that developed the current Cisco IOS user interface under contract to Cisco Systems. Terry is experienced in the design and installation of large TCP/IP based networks and is a successful network protocol instructor. He is the second Cisco Certified Internetworking Expert (CCIE) #1026 and the first outside of Cisco.
I've done a number of network assessments in which I find that the customer insists on manually configuring the interfaces for full duplex. I did a little research on the topic and found an interesting paper by Jim Eggers and Steve Hodnett at Sun Microsystems (http://www.sun.com/blueprints/0704/817-7526.pdf). It is from 2004 and while that's a few years ago, it shows what the accepted practice was back then.
Of particular interest to me was the section titled "Loss of Functionality and Capabilities," which describes a set of problems that can occur when auto duplex is disabled. Of course, there are the problems and packet loss that occur if you have a duplex mismatch. But of particular interest is how the lack of duplex sensing may affect the ethernet interface's ability to detect signal quality or to quickly detect link failures.
At a recent Cisco seminar, the presenter recommended that auto-negotiate be used, particularly on fiber interfaces, for fast link failure discovery. Let's say that two systems, A and B are connected to each other and that the cabling is damage in such a way that it becomes uni-directional (system A is receiving from system B, system B is not receiving from system A) System B will begin the negotiation process towards system A, which tells A that the link is now uni-directional. This information can now be quickly propagated up the stack so that the higher level protocols can react and hopefully select an alternate path around the problem.
The state of the technology has certainly improved since 2004. Cisco's recommendation is to use auto-duplex. The problems in the early days of auto-negotiation are behind us, so I recommend that anyone who is still hard-coding duplex take a look at it and think about whether it is a good practice to continue to hard-coding duplex.
-Terry
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