The Importance of Configuration and Operational Views of the Network

Terry Slattery
Principal Architect

Network configurations are a valuable source of information about the intent of a network’s design, but just analyzing configurations is not sufficient for detecting and resolving common network problems.  A set of configurations may tell one story while the operational state of the network tells a different story.  Let’s look at an example: spanning tree configuration.  The great thing about transparent bridging (which uses spanning tree) is that devices just work when connected to the network (it’s transparent to the device).  The bad thing about such networks is that when there’s a problem, the transparency can make it difficult to determine what’s really happening.  Let’s say that you have two switch configurations and each configuration contains the following (using Cisco syntax):

vlan 411
  name VoiceVLAN
  spanning-tree vlan 411 priority 8192

All by itself, you might say that the switch would be the root bridge in VLAN 411 (the Voice VLAN). Let’s look at two scenarios whose outcomes depend on the operational intent of the installation of a second switch that uses the same confguration.

Scenario 1:

After the deployment in the network, someone adds another switch to the VLAN and copies the above configuration, forgetting to change the priority.  Now you have two switches in the VLAN with the same priority.  The lowest MAC address will be used to select the root bridge, potentially creating an undesirable topology.  If the second switch has limited capacity, it may cause the VLAN to fail or become unstable.

Scenario 2:

The second switch, using the identical configuration, is installed in a different VLAN.  This is a valid configuration and everything is working correctly.  However, just checking the configuration against best practices or other configurations won’t tell you whether it is correct or faulty.

There are multiple ways to check the configuration of a new device to determine if its configuration will cause a problem when it is added to the network.  The ‘guess and test’ approach is to install the new device and collect the operational data that tells you if two bridges have the same priority and which one has become the root bridge.  In scenario one above, the operational data would show that two switches in one VLAN had the same priority and that the root bridge was selected by lowest MAC address.  In scenario two above, the operational data would tell you that the two devices were in separate subnets/VLANs and that the configurations were valid.

Another way to check the configurations is to manually provide some meta-data prior to installing the devices.  This meta-data would require the identification of devices that would be operational neighbors of the new device (in this case other switches that will be in the same VLAN).  It may be necessary to identify where the new device would be connected for the configuration to be correctly  validated.  There is a small amount of additional effort required, but that is a small price to pay to keep the network running smoothly.

There are similar scenarios in various technologies.  Take the case of OSPF in determining the router ID of a virtual link.  A new configuration may add a loopback interface, which would then change the OSPF Router ID, causing any virtual link that used the old router ID to fail.  Here, configurations of a neighboring device (the other end of the virtual link in this case) depends on the existance of a loopback interface in the configuration.

In summary, configuration analysis must incorporate operational data and the configurations of neighboring devices in order to accurately determine the correctness of a device’s configuration.



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.