Cisco NAC Troubleshooting Tip: SNMP Write Not Working


When Cisco NAC is used in out-of-band (OOB) mode, SNMP read and write functionality is critically important.  The NAC Manager uses the SNMP write community to change the VLAN that individual switch ports are connected to.  For example, after a user passes NAC authentication and posture assessment, the end computer is moved from the authentication VLAN to the access VLAN.  If something stops this SNMP write packet from getting from the NAC Manager to the switch, then NAC functionality would be broken.  

There is one IOS configuration command that could cause sporadic outages.  This command is “configuration mode exclusive”.  This command allows only one session to be in “configuration terminal” mode at a time.  This is a great feature for making sure that two users are not making conflicting changes at the same time.  It is a horrible feature to enable with NAC.  The problem exists where a user is logged into an access switch in “configuration terminal” mode.  If the user is in this mode and the NAC Manager sends an SNMP set command, the SNMP set command will fail.  The indication will be that the computers, connected to access switches, are not successfully moved from the authentication VLAN to the access VLAN after successful NAC authentication and posture assessment.  The logs on the NAC Manager in /perfigo/control/tomcat/logs/nac_manager.log should show information related to failed communication with the end switch.  

Troubleshooting this problem is frustrating because the problem is only seen sporadically.  If there are no sessions in “configuration terminal” mode, then the NAC Manager will work correctly.  It is only when a “configuration terminal” mode session exists that the NAC Manager SNMP set command fails.  

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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.