EIGRP Query Process

🧠 EIGRP Route Selection & Query Process

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1. ✅ Feasibility Condition (FC)

To qualify as a Feasible Successor (FS), a neighbor’s route must meet the Feasibility Condition:

Reported Distance (RD) < Feasible Distance (FD)

• Ensures loop-free backup routes.
• FS routes are stored in the topology table, not routing table.

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2. 📘 Key Definitions

Term	Description
Successor (S)	Primary/best path to a destination (added to routing table).
Feasible Successor (FS)	Backup route meeting FC (stored in topology table only).
Feasible Distance (FD)	Best known distance from the router to the destination.
Reported Distance (RD)	Distance reported by a neighbor to reach the destination.
Passive State	Route is stable (normal state).
Active State	Router is actively searching for a new path by sending queries.

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3. 🗺️ Topology Example (Based on Your Diagram)

Destination: 192.168.10.0/24
Source Router: R1

Path	Next-Hop	Status
Path 1	R2	✅ Successor
Path 2	R3	❌ Not FS (doesn’t meet FC) — not stored
Path 3	R4	✅ Feasible Successor (FS)

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4. 🚨 Failure Scenario Walkthrough

🔹 Step 1: Successor (R2) Fails

• FS via R4 gets promoted to new Successor
• Route remains in Passive state (no queries needed)
• Still stable and reachable

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🔹 Step 2: FS (R4) Also Fails

• Now R1 has:
  • ❌ No Successor
  • ❌ No Feasible Successor
• R1 changes route to Active State
• Sends Query messages to:
  • R3 (even though it wasn’t FS)
  • Any neighbor that may have a valid path

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5. 🔄 EIGRP Query & Reply Mechanism

Phase	Action
R1 sets the route to Active State	Because no FS is available
R1 sends Query to neighbors	“Do you have a route to 192.168.10.0/24?”
Neighbors check their own routing/topology tables
If they have a valid route → Reply
If not → Forward Query to their own neighbors
R1 waits for Replies from all queried neighbors

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6. ⏱️ SIA (Stuck-in-Active) Process

⚠️ What is SIA?

A route becomes Stuck-in-Active when a router:

• Sends a query
• But doesn’t get replies from all neighbors in time

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🧩 SIA Timer Mechanics

Timer	Purpose
180 sec total	Time router waits before declaring route as SIA
First 90 sec	Router may send SIA-Query to unresponsive neighbor
SIA-Reply	If neighbor responds to SIA-Query, original 180 sec is reset

🔁 So the flow is:

1. Query sent
2. If no reply within 90 seconds, router sends SIA-Query
3. If neighbor responds with SIA-Reply, route stays active, timer resets
4. If still no reply after full 180 seconds, the route is marked SIA
5. Router:
   • Drops the route
   • Possibly resets the neighbor adjacency

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7. ❗ Key Point: Passive vs. Active

Route State	When It Happens
Passive	Normal state – route is stable & reachable
Active	Triggered only when: no S & no FS
	Router sends Query messages

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8. 🧠 Final Recap

Event	R1 Behavior
R2 (Successor) fails	R4 (FS) becomes new Successor; state remains Passive
R4 also fails	No S/FS → Route becomes Active
R1 sends Queries	To all neighbors asking for alternative routes
No reply in 90 sec	Sends SIA-Query
Reply received	Route remains Active; timer resets
No reply in 180 sec	Route becomes Stuck-in-Active, removed from topology + routing table
Neighbor unresponsive	Adjacency with neighbor may be reset

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