Troubleshooting Redundancy in OSPF

Author
Carole Warner Reece
Architect

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.

Background
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.

2011_10_11_basic_topology

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.

2011_10_11_ospf_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.

Summary
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

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

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