One of the most misunderstood behaviors in EIGRP is the “Stuck-In-Active” (SIA) condition. This occurs when a router does not receive a reply to its query within the expected timeframe. As networks grow in complexity, especially with discontiguous topologies and no summarization, SIA becomes more common and disruptive.
Understanding SIA
When a route goes down and no feasible successor exists, EIGRP sends queries to neighbors. If a query is not acknowledged in time, the originating router marks the route as SIA and drops it. This can result in unnecessary route loss and network churn.
Scenario: Poor Query Boundary Design
In this scenario, an enterprise network has EIGRP enabled across many routers, but lacks properly configured summarization or route filtering. A single route failure leads to queries being flooded across dozens of routers, increasing the risk of delay and SIA events.
Configuration Walkthrough: Applying Stub Routing
router eigrp 100
eigrp stub connected summary
By configuring EIGRP stub routing on access routers, you limit the query scope. This prevents those routers from forwarding queries back toward the core, reducing convergence time and eliminating SIA conditions.
Verification and Diagnostics
debug eigrp fsm
show ip eigrp topology
show ip protocols
Use these commands to identify where queries originate and whether replies are received. The FSM debug will show transitions that indicate query/reply flow. Check for missing replies or long round-trip delays.
Conclusion
EIGRP SIA issues are avoidable with proper topology design. Always define summarization, apply stub routing at network edges, and monitor convergence events. This allows networks to scale predictably and minimizes the risk of EIGRP instability in large environments.
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