Finding Devices To Manage

Terry Slattery
Principal Architect

Remember the old management adage “You can only manage what you can measure”?  Well, there’s a corollary in Network Management, in which you can only manage devices that you know to exist.  In my current job, the network has over 600 core network devices (e.g. routers and switches) and in the entire network, there are over 4000 network devices.  How do we know that the network management platform has access to all the key devices?  Well, we needed a way to check.

I built a report in NetMRI today that would tell us which network devices have been discovered, but are not being managed.  The process requires a good network discovery as its foundation.  I’ve used a variety of network management tools and NetMRI’s discovery is one of the best I’ve seen.  It relies on multiple sources of data, such as ARP, switch forwarding tables, CDP, and routing neighbors, to discover neighboring devices, then tries to discover the SNMP and login credentials to use for each device.  The security team should be notified prior to running NetMRI on the network because any SNMP and login attempt failures should create a security alarm.  (If it doesn’t, then the security team should be concerned.)  Once SNMP access is obtained, neighbor information from the newly discovered device is used to expand the discovery.

How do you find out about devices where SNMP access was not successful?  Hopefully, you have a standard way to identify your networking devices, such as having known device management address ranges or known device naming standards.  You may need to use a combination of these approaches to identify the network devices.  Fortunately for me, the naming standards where I’m working made it easy to identify network devices.  So I built a report that identifies devices whose DNS name matches the known naming pattern.  In this network, the naming pattern used a site ID, so the report filter was:

1: Devices: DNS Name matches /^[0-9][0-9][0-9]-/

The matching pattern defines a regular expression (the text between the ‘/’s) that matches beginning of the name (‘^’) and then three digits ([0-9][0-9][0-9]), followed by a hyphen.  This will match any device name that starts with NNN- (where N is a digit).

I then needed a way to test whether NetMRI had SNMP access to each device.  NetMRI won’t increase the device assurance above 75 without having SNMP access, so I added a second filter rule:

2: Devices: Assurance < 90

The two filter rules were combined by a logic definition that required both conditions to be matched:

Logic: 1 and 2

This report did the job and quickly identified about ten devices that had been discovered but which NetMRI could not access.  These devices didn’t have the correct SNMP configuration, and having the list made it easy to quickly correct them.



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.