Troubleshooting Redundancy in OSPF

Carole Warner Reece

I was talking to some network engineers at an organization I will call Charlie. They thought they had a redundant network, but after looking at their network I believe it will not quite work as they expect.

The physical topology in the section I was reviewing has some redundancy. The R5 devices are in one building, the R3 devices in another. A MAN link connects the two buildings. Several remote sites are connected to the WAN routers through T1 links. The CORE devices are connected to OSPF Area 0 which connects to the rest of the Charlie network.


After looking at their configuration files, I sketched out the following diagram to illustrate the affected part of the network in terms of their OSPF routing topology.


The links in the network diagram are color coded to show which area they are in.

What do you see as potential issues and concerns?

My Observations
Their network is not redundant at Layer 3. There are several single points of failure that can isolate all of the remote WAN sites from the rest of the network:

  • The uplink from R3-core1 to Area 0 (Gi0/2 on R3-core)
  • The uplink from R3-wan1 to R3-core1
  • A failure of R3-core 1 or R3-wan1

Since the link from R5-wan1 to R5-dist is not in an OSPF area, R5-wan1 has to send inter-area traffic across the MAN link to reach the rest of the network.

In addition, if the link between R5-wan1 and R3-wan1 fails, Remote1 will blackhole traffic to the rest of the network, since Remote1 will continue send inter-area traffic from the local Area 100 to R5-wan1.

I am also not sure that having four separate regions of Area 100 was planned.

OSPF Layer 3 redundancy does not follow Layer 1 redundancy. When designing and troubleshooting networks, you need to keep in mind how OSPF areas function. As I discussed in Discontiguous non-Area 0 Areas in OSPF, while discontiguous non-Area 0 areas can be supported as a short term measure, this is not a good long term design for production networks.

To add Layer 3 redundancy to Charlie’s network and improve the design, I would recommend at least two changes:

  • Move the Gi0/1 from R5-wan directly to r5-core, and place it in Area 0
  • Move all the serial links from the WAN routers into Area 100.

— cwr


Related Blogs
Other NetCraftsmen blogs on OSPF design or redundancy include:

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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

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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.