When EIGRP loses a successor with no feasible successor, it queries neighbors. Each neighbor queries its neighbors. The route is "active" during this process.
Problem: If any router doesn't reply, the query hangs. After minutes without a reply, the neighbor relationship resets. This is Stuck-in-Active (SIA).
Causes:
- Slow router under heavy load
- Unidirectional link failure
- Query scope too large
Solutions:
- Summarize routes to limit query scope
- Use stub routers for branches
- Ensure bidirectional connectivity