Understanding Route Redistribution: Avoiding Routing Loops and Black Holes

February 2015   |   Reading Time: 10 min read

Introduction

Route redistribution is a vital mechanism in multi-protocol networks, allowing routing information to be shared across different routing domains. Whether integrating OSPF with EIGRP, BGP with static routes, or connecting disparate administrative boundaries, redistribution provides the glue. However, it can also create routing loops, suboptimal paths, or black holes if not handled carefully.

What is Route Redistribution?

At its core, redistribution is the process of taking routes learned via one routing protocol and injecting them into another. This is useful in hybrid networks or during migrations. For example, redistributing EIGRP routes into OSPF ensures that OSPF routers learn about EIGRP networks.

Risks of Improper Redistribution

Without careful design, redistribution can result in routing loops, where packets circulate endlessly, or black holes, where packets are dropped. These issues are more likely in mutual redistribution scenarios (where both protocols redistribute into each other).

• Unreachable subnets
• Increased CPU due to route churn
• Duplicate routes with different metrics
• Loss of deterministic routing behavior

Key Design Principles

• Prefer one-way redistribution whenever possible.
• Use route tagging to prevent loops.
• Apply filtering to control which routes are injected.
• Leverage administrative distance to prefer internal protocols.
• Avoid redistributing default routes unless necessary.

Sample Configuration: EIGRP into OSPF

    router ospf 1
     redistribute eigrp 100 subnets route-map EIGRP-TO-OSPF
    !
    route-map EIGRP-TO-OSPF permit 10
     match tag 100
     set metric 1000 1 255 1 1500
    !
    router eigrp 100
     redistribute ospf 1 metric 10000 100 255 1 1500 route-map OSPF-TO-EIGRP
    !
    route-map OSPF-TO-EIGRP permit 10
     set tag 100

Mutual Redistribution and Route Tagging

Mutual redistribution (both protocols redistributing into each other) increases the chance of routing loops. The only effective solution is to use route tags. By tagging routes as they exit a protocol and filtering those tags on reentry, you can prevent re-injection of already redistributed routes.

Using Administrative Distance

When the same prefix is learned from two different protocols, the router chooses the one with the lowest administrative distance. Understanding and tuning administrative distances can ensure protocol preference and provide loop protection.

Redistribution with Static and Connected Routes

Redistribution isn’t limited to dynamic protocols. Connected and static routes can also be redistributed. Ensure you use appropriate route-maps to avoid unintentionally injecting every connected route into your IGP.

Troubleshooting Tips

• Use show ip route to examine route sources and metrics.
• Use debug ip routing sparingly to see real-time route changes.
• Tag and track redistributed routes.
• Use traceroute to detect loops or path inconsistencies.

Common Pitfalls

• Forgetting to match subnets when redistributing into OSPF.
• Allowing the default route to propagate into unwanted domains.
• Using mutual redistribution without route tagging.
• Neglecting to adjust administrative distance when needed.

Conclusion

Route redistribution is essential in real-world networks, but it demands care and planning. By using tags, controlling metrics, and applying filters, engineers can avoid the most dangerous pitfalls. Testing in a lab and monitoring closely in production will ensure routing behavior remains predictable and loop-free.

Eduardo Wnorowski is a network infrastructure consultant and technologist.
With over 20 years of experience in IT and consulting, he brings deep expertise in networking, security, infrastructure, and transformation.
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