Auto-negotiate Duplex or not?

Terry Slattery
Principal Architect

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 (Ethernet Autonegotiation Best Practices).  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 damaged 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.



Re-posted with Permission

NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under



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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.