Feasible Distance (FD)
Foundation of DUAL
Feasible Distance is the metric of the best path to a destination. It's the cornerstone of DUAL's loop prevention mechanism and route selection process.
What is Feasible Distance?
Feasible Distance (FD) is the lowest calculated metric among all paths to a destination since the last time the route went from Active to Passive state. It represents the best known distance to reach a destination network.
Feasible Distance Characteristics
- Best Path Metric: FD is the metric of the successor route
- Historical Reference: FD is the lowest metric ever recorded for this destination
- DUAL Anchor: Used to evaluate feasibility of alternate paths
- Loop Prevention: Prevents counting to infinity scenarios
Feasible Distance Lifecycle
FD Evolution Process
Initial Route
First route learned
FD = Initial metric
Better Route
Lower metric found
FD = New lower metric
Route Loss
Successor fails
FD remains unchanged
New Successor
New best path found
FD = New successor metric
FD vs Current Distance
| Metric Type | Definition | When Updated | Purpose |
|---|---|---|---|
| Feasible Distance (FD) | Lowest metric ever recorded | Only when a better path is found | Loop prevention reference |
| Current Distance | Current successor's metric | Whenever successor changes | Active route metric |
Feasible Successor
Backup Path Insurance
Feasible Successors are loop-free alternate paths that can be immediately used when the primary path fails, enabling sub-second convergence in EIGRP networks.
What is a Feasible Successor?
A Feasible Successor is a neighboring router that has a loop-free alternate path to a destination. It must satisfy the feasibility condition: the neighbor's Reported Distance (RD) must be less than the current Feasible Distance (FD).
Feasibility Condition Formula
RD < FD
Where:
- RD: Reported Distance from the neighbor
- FD: Current Feasible Distance to destination
Successor vs Feasible Successor
| Aspect | Successor | Feasible Successor |
|---|---|---|
| Definition | Best path to destination | Loop-free alternate path |
| Quantity | One per destination | Zero or more per destination |
| Routing Table | Installed in routing table | Stored in topology table only |
| Traffic Flow | Actively forwards packets | Standby, ready for promotion |
| Promotion | N/A | Becomes successor when needed |
Route Computation
DUAL Route Computation
Route computation in EIGRP involves the sophisticated DUAL algorithm that ensures loop-free routing while maintaining optimal paths and fast convergence.
DUAL Route Computation Overview
DUAL (Diffusing Update Algorithm) performs route computation by evaluating paths based on feasibility conditions, maintaining loop-free topology, and enabling rapid convergence through local computations when possible.
Route Computation Process
Step-by-Step Route Computation
Input Processing
Receive route
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Metric Calculation
Calculate total
path metric
Feasibility Check
Apply feasibility
condition
Route Selection
Choose best path
and alternates
Local vs Global Computation
| Computation Type | Trigger | Process | Convergence Time |
|---|---|---|---|
| Local Computation | Feasible successor available | Promote FS to successor | Sub-second |
| Global Computation | No feasible successor | Query/reply process | Seconds to minutes |
Loop Prevention
EIGRP's Loop-Free Guarantee
EIGRP's DUAL algorithm provides mathematically proven loop-free routing through the feasibility condition and sophisticated distance vector mechanics.
Why Loop Prevention Matters
Routing loops can cause:
- Infinite packet forwarding - Packets bounce between routers
- Network congestion - Bandwidth consumed by looping traffic
- Resource exhaustion - CPU and memory overload
- Service outages - Legitimate traffic cannot reach destinations
The Feasibility Condition
Mathematical Foundation
RD < FD
A route is loop-free if the neighbor's Reported Distance is less than our Feasible Distance
Why the Feasibility Condition Works
Loop Prevention Logic
Assumption
Neighbor's RD < Our FD
Logical Deduction
Neighbor is closer
to destination
Conclusion
No path through us
to destination
Result
Loop impossible
Loop Prevention Mechanisms
| Mechanism | Description | Implementation | Effectiveness |
|---|---|---|---|
| Feasibility Condition | RD < FD check | Automatic in DUAL | 100% loop prevention |
| Split Horizon | Don't advertise back to source | Automatic | Prevents immediate loops |
| Query Scoping | Limit query propagation | Stub configuration | Reduces loop potential |
| Active Timer | Timeout stuck computations | SIA detection | Recovers from loops |
Key Takeaways
- Feasibility Condition: RD < FD guarantees loop-free routing
- Mathematical Proof: DUAL provides provable loop freedom
- Practical Implementation: Automatic in EIGRP, no configuration needed
- Performance Balance: Loop prevention with fast convergence