Loop-free path selections (RD, FD, FC, successor, feasible successor, stuck in active)

1.9 Troubleshoot EIGRP (classic and named mode; VRF and global)

📘CCNP Enterprise – ENARSI (300-410)

EIGRP Loop-Free Path Selections

EIGRP (Enhanced Interior Gateway Routing Protocol) is a dynamic routing protocol used in networks to determine the best paths for traffic. One of its key strengths is ensuring loop-free paths — meaning data won’t circulate endlessly between routers.

To understand loop-free paths in EIGRP, you need to know these terms:

1️⃣ RD – Reported Distance

Definition: The distance a neighbor router reports to reach a particular network.
Key point: It’s from the neighbor’s perspective, not yours.
IT example: If Router A asks Router B how far Network X is, Router B says “10.” That “10” is the RD for Router A.
Exam relevance: RD is used to calculate if a route is loop-free.

2️⃣ FD – Feasible Distance

Definition: The lowest total metric (distance) from your router to a destination network.
Key point: FD = best path cost known from your perspective.
IT example: If Router A sees two paths to Network X (via Router B = 10, via Router C = 15), the FD is 10, the best metric.
Exam tip: FD is used to identify the successor, the primary path.

3️⃣ FC – Feasible Condition

Definition: The rule EIGRP uses to determine if a path is loop-free and can become a feasible successor.
Formula: Reported Distance (RD) < Feasible Distance (FD of current successor)
Meaning: If a neighbor reports a distance smaller than your best path, it’s a safe backup route.
IT example:
- Current best path to Network X (FD) = 10
- Neighbor reports (RD) = 8 → satisfies FC → can be used as feasible successor
- Neighbor reports (RD) = 12 → does NOT satisfy FC → cannot be a backup

4️⃣ Successor

Definition: The best path to a network (lowest FD).
Role: The route actually installed in the routing table and used to forward packets.
IT example: Router A has two paths to Network X:
- Via Router B: metric 10
- Via Router C: metric 15
  → Router B’s path is the successor (FD = 10).

5️⃣ Feasible Successor

Definition: A backup path that satisfies the feasible condition (FC).
Key point: Only used if the successor fails. Ensures loop-free backup routing.
IT example: Router A sees:
- Successor: Router B (metric 10)
- Backup: Router C (RD 8, metric 15) → satisfies FC → stored as feasible successor
- Backup: Router D (RD 12, metric 12) → does NOT satisfy FC → ignored
Exam tip: Always check RD < FD to determine feasible successors.

6️⃣ Stuck in Active (SIA)

Definition: A condition where EIGRP cannot find a successor or feasible successor in a timely manner after a network goes down.
Cause:
- No feasible successor exists
- Neighbor routers do not respond
Impact: Route stays in active state, waiting for replies → can cause delays.
IT example:
- Network X goes down
- Router A asks neighbors for a new route
- Neighbor responses are delayed → Router A is stuck in active
- Once a reply comes or a new path is found, it transitions back to normal.
Exam tip:
- SIA is critical for troubleshooting.
- Commands: show ip eigrp topology → check active routes.

How EIGRP Uses These for Loop-Free Routing

Select Successor: Pick the lowest FD → the main path.
Check Feasible Condition: See if any neighbor has RD < FD → store as feasible successor (backup).
Forward Traffic: Use successor in the routing table.
Failover: If successor fails, immediately switch to feasible successor (loop-free).
If No Backup: Network goes active → query neighbors → may enter SIA temporarily.

Commands for Exam / Troubleshooting

show ip eigrp neighbors → Verify neighbor relationships
show ip eigrp topology → Shows FD, RD, successor, feasible successor
show ip route eigrp → Check routes installed in the routing table
debug eigrp packets → Monitor real-time route updates

Exam Summary / Key Points

RD → neighbor-reported distance
FD → your best path distance
FC → RD must be < FD for backup path
Successor → best path, installed in routing table
Feasible Successor → backup path, loop-free, ready if successor fails
Stuck in Active (SIA) → occurs if no loop-free path is available and replies are delayed

Tip: Focus on understanding the relationship between RD, FD, FC, successor, and feasible successor, because the exam will often test your ability to identify loop-free backup paths.