Routing Loops

Terry Slattery
Principal Architect

I’m surprised by people who don’t understand how they could create routing loops with static routes.  These people seem to be really surprised that it happened.  Upon investigation, they don’t seem to understand dynamic routing and prefer to use static routes.

Maybe I’ve been lucky and not enjoyed the thrill of creating a routing loop and then troubleshooting it. Wink
I consider static routes to be a bad idea except in special cases.  I only use them at the edge of the network where using route redistribution could be even more dangerous or complex.  By limiting static routes to specific cases at the edge, I can control where they point and avoid problems.  An example is a static route via a back-door link to a partner’s network.  A corresponding reverse route is needed, or two-way connectivity won’t exist.  Both parties need to agree on the routing mechanism to be used to avoid problems.  Very seldom do I want to import routes from a partner’s routing protocol.  Weirdness within their routing domain may impact my router, so a static route insulates my network from theirs.

My bogon alert goes off whenever someone suggests that a static route point to another part of the IGP domain.  It is generally a sign that we need to review the IGP design and figure out why it isn’t doing what we want.  I call it “band-aiding the network,” because it is applying a patch to the network when someone didn’t want to take the time to understand why the existing system didn’t have the desired connectivity.

Another interesting scenario occurs when a static route exists, and everything works until a seemingly unrelated topology change creates a routing loop.  The cause is often because the IGP, or another static route, is now pointing back along the path of the initial static route.  So the initial change could be weeks or months removed from the failure.

A good analogy is in the software maintenance arena.  The most common source of bugs is due to the maintainer not reading and understanding what the existing code does before implementing a change.  Understanding network configurations is similar to reading a program’s source code.  It is often tempting to slap in a “quick fix,” declare the task closed, and move on to something else.  But in too many cases, the fix has bad side effects and has to be revisited.  If there is no revision tracking (does your network management system collect config revisions?), you have to try to figure out what was changed, then figure out what the right fix should be.   A history of configuration changes is needed if the change was implemented long ago and another seemingly simple change triggered the problem.

How have static routes helped or hurt your network?



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.