Proper Etherchannel Configuration

Terry Slattery
Principal Architect

Rick Burts (CCIE #4615) of Chesapeake NetCraftsmen told several of us about an interesting problem he recently encountered.  Here’s his description of the problem:

This week I was called to assist [a customer] with a problem in their network. The problem was tricky and very intermittent. It impacted some users but not others. Impacted users could access certain destination addresses but could not access other destination addresses within the same subnet.

It felt like a routing issue. But since some addresses within a subnet were reachable while others within the same subnet were not reachable it was hard to see how routing was the issue. After extensive troubleshooting we diagnosed that it was a problem within an Etherchannel configured between a couple of switches. Each switch was configured with four ports in the Etherchannel. One switch showed three ports active in the Etherchannel and one port not connected. The other switch showed two ports active in the Etherchannel and two ports not connected.

So the problem was that one switch saw a port as up while the other switch saw the port as down. No one was sure how the port was marked as not connected. The problem was exacerbated by the fact that all ports in the Etherchannel were configured with mode on and by the fact that UDLD was not configured. We got the port back to an upstate and connectivity was restored for all destination addresses. We also configured the ports to mode desirable and enabled UDLD. It was a very interesting problem to figure out.

The core of the problem was that the switches were not configured properly.  If the customer didn’t know the proper switch configuration to use, it is unlikely that they would have built a configuration policy check would have found the error.  That’s where policies based on the Cisco SRNDs (Solutions Reference Network Design guides) are useful.

And let’s say that the network engineers didn’t have a system to check network configuration policy or that they weren’t aware of the current configuration best practices for Etherchannel.  A network analysis system would have identified that  some links in the Etherchannel were not operational, potentially identifying the problem before the users were affected.  This is an example of common problems that senior network engineers know that they should check, but they never have the time to proactively seek.  That’s where automated network analysis, of both configurations and operational data, is important.



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.