The EIGRP Feasibility Condition (FC) is the core loop-avoidance mechanism for EIGRP, a popular Cisco-proprietary distance vector protocol. On the CCNA 200-301 exam, understanding the FC is critical for troubleshooting EIGRP convergence, interpreting topology tables, and answering scenario-based questions about route selection. In real networks, the FC ensures fast failover with zero loops, making EIGRP a go-to choice for many enterprises. Exam objective 3.5 covers EIGRP operation, including the feasibility condition and successor selection.
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Imagine you're a delivery driver in a city with a constantly changing road network. Your dispatcher gives you a map book with every possible route to every destination. However, to avoid traffic jams and loops, you have a strict rule: you can only use a route suggested by another driver if that driver's distance to the destination is less than your current best distance. This prevents you from taking a detour that circles back. For example, you know the shortest route to the airport is 10 miles. A fellow driver reports a route via their neighborhood that is 12 miles from them to the airport. Since 12 is greater than your 10, you reject it—that route might loop back through your area. But if another driver says their distance is 8 miles, you accept their route because it's shorter than yours, guaranteeing no loop. This rule—only accept a neighbor's route if their metric is less than your own feasible distance—is the Feasibility Condition. It ensures you always take a path that is strictly closer to the destination, preventing routing loops even when the network changes. Similarly, in EIGRP, a router only installs a feasible successor route if the reporting neighbor's advertised distance (AD) is less than the router's own feasible distance (FD) to that destination.
What is the Feasibility Condition?
The EIGRP Feasibility Condition is a loop-prevention mechanism used during Diffusing Update Algorithm (DUAL) operation. It determines whether a route reported by a neighbor can be used as a backup (feasible successor) without creating a routing loop. The condition is simple: a route is feasible if the neighbor's advertised distance (AD) to the destination is strictly less than the local router's feasible distance (FD) for that destination. The FD is the lowest metric ever computed for that destination since the last topology change. The AD is the metric the neighbor claims it costs to reach the destination.
Why Does It Exist?
In distance vector protocols, routers rely on neighbors for reachability information. Without a loop-avoidance mechanism, a router could accept a route that leads back to itself, creating a loop. EIGRP's DUAL algorithm uses the Feasibility Condition to ensure that any backup route is guaranteed to be loop-free. This allows EIGRP to converge rapidly—often in sub-second time—without the periodic updates of RIP or the complex LSA processing of OSPF.
How It Works Step by Step
Initial Learning: Router R1 receives an EIGRP update from neighbor R2 for network 10.1.1.0/24. The update includes a metric (the AD). R1 computes its own metric to reach 10.1.1.0 via R2: Metric = AD + cost(R1-R2). This computed metric becomes the FD if it is the best (lowest) metric for that destination.
Feasible Distance (FD): R1 records the FD as the lowest metric it has ever had for 10.1.1.0/24. Initially, this is the metric via R2.
Successor Selection: The route with the lowest metric (the successor) is installed in the routing table. R1's successor for 10.1.1.0/24 is via R2, with metric = FD.
Feasible Successor Check: Later, R1 receives another update from neighbor R3 for the same network. R3's AD is 2000. R1 compares R3's AD to its current FD (say 3000). Since 2000 < 3000, the Feasibility Condition is met. R3's route is a feasible successor. R1 stores it in the topology table as a backup.
What If AD >= FD?: If R3's AD were 3500 (>= 3000), the condition fails. R1 cannot use R3 as a feasible successor because the route might loop. EIGRP will not install it as a backup; instead, it will be marked as an active route if the successor fails, and DUAL will query neighbors to find a new path.
Key States, Timers, and Defaults
FD (Feasible Distance): The best metric ever for a destination since last change. Stored per route.
AD (Advertised Distance): The metric reported by a neighbor for a destination.
Successor: The next-hop router with the lowest metric (FD) for a route. Installed in routing table.
Feasible Successor (FS): A backup route that meets the FC. Stored in topology table. Can be used immediately if successor fails.
DUAL States: Passive (stable) vs Active (querying for new path). If no FS exists, route goes Active.
Timers: No specific timer for FC; DUAL uses hold timers (default 15 seconds) and retransmission timers (RTO) for reliable updates.
IOS CLI Verification
To see the FD, AD, and feasible successors, use show ip eigrp topology:
R1# show ip eigrp topology
P 10.1.1.0/24, 1 successors, FD is 3072
via 192.168.1.2 (3072/2816), GigabitEthernet0/0
via 192.168.2.2 (3328/3072), GigabitEthernet0/1FD is 3072: The feasible distance.
(3072/2816): The first number is the computed metric via that neighbor (the total cost). The second number (2816) is the AD reported by the neighbor.
The first entry (via 192.168.1.2) is the successor (metric = FD). The second entry is a feasible successor because its AD (3072) is less than FD (3072? Wait, AD = 3072, FD = 3072 — condition requires strictly less. In this output, AD = 3072, FD = 3072, so the condition is NOT met? Actually, the condition is AD < FD. Here AD = 3072, FD = 3072, so it is NOT less. But the output shows it as a feasible successor? That would be incorrect. In reality, the condition is AD <= FD? No, it's strictly less. However, Cisco's implementation sometimes allows equality? The CCNA exam expects strictly less. So if AD == FD, it is not a feasible successor. The output above might be contrived; in practice, AD must be strictly less.
Let's correct: In a real scenario, the second entry would have AD = 2816, which is < 3072, making it a feasible successor.
Interaction with Other Protocols
EIGRP can redistribute routes from other protocols (OSPF, static). The FC applies to all routes in the EIGRP topology table, regardless of source. Redistribution metrics must be set carefully to ensure FC works correctly and avoid suboptimal routing.
Summary
The Feasibility Condition is a simple yet powerful loop-avoidance rule: AD < FD. It allows EIGRP to maintain loop-free backup routes, enabling fast convergence without the overhead of periodic updates.
Understand Feasible Distance (FD)
Feasible Distance (FD) is the best metric a router has ever computed for a destination since the last topology change. It is stored per route in the topology table. For example, if R1's best path to 10.1.1.0/24 has metric 1000, then FD = 1000. The FD is used as a reference point for the Feasibility Condition. When a new route is learned, its Advertised Distance (AD) is compared to the FD. If AD < FD, the route is loop-free and can be used as a backup. The FD only changes when the successor route is removed or a better path is found.
Learn Advertised Distance (AD)
Advertised Distance is the metric that a neighbor reports for a destination. It represents the neighbor's cost to reach that network. For example, if R2 advertises 10.1.1.0/24 with metric 500, then the AD from R2 is 500. The local router adds the cost to reach R2 to get the total metric. The AD is critical for the Feasibility Condition: the router checks if the AD is less than its own FD. If yes, the route is feasible. If no, the route is not a feasible successor and may cause a loop if used.
Apply the Feasibility Condition
The Feasibility Condition is: AD < FD. If a neighbor's AD is strictly less than the local router's FD for that destination, the route is considered loop-free and can be used as a feasible successor. This ensures that the neighbor is closer to the destination than the local router, so any path through the neighbor will not loop back. For example, if FD = 1000 and a neighbor reports AD = 800, the condition is met. If AD = 1000 or more, it fails. The condition is checked for every update received.
Identify Successor and Feasible Successor
The successor is the next-hop router with the lowest metric (the FD). It is installed in the routing table. A feasible successor is a backup route that meets the Feasibility Condition. It is stored in the topology table but not in the routing table unless the successor fails. For example, if the successor metric is 1000 (FD=1000) and another neighbor reports AD=800, that neighbor is a feasible successor. If the successor goes down, the router immediately uses the feasible successor without any queries, achieving fast convergence.
Verify with show ip eigrp topology
Use the command `show ip eigrp topology` to view FD, AD, successors, and feasible successors. The output shows each route with its successors and feasible successors. For example: ``` R1# show ip eigrp topology P 10.1.1.0/24, 1 successors, FD is 3072 via 192.168.1.2 (3072/2816), GigabitEthernet0/0 via 192.168.2.2 (3328/3072), GigabitEthernet0/1 ``` The first number in parentheses is the total metric, the second is the AD. Here, the successor is via 192.168.1.2 with total metric 3072 and AD 2816. The second route has total metric 3328 and AD 3072. Since AD (3072) is equal to FD (3072), this route does NOT meet the Feasibility Condition (strictly less required). So it is not a feasible successor. The output would show it as a feasible successor only if AD < FD. In this case, it is just an alternate route but not loop-free.
Troubleshoot FC Failures
If a route shows as active (not passive) in `show ip eigrp topology`, it means no feasible successor exists, and DUAL is querying neighbors. This can happen when the FD is very low and all ADs are higher. To fix, you can adjust metrics (e.g., using delay or bandwidth) to make some ADs lower than the FD. Alternatively, you can clear the topology table with `clear ip eigrp topology` to reset FD. On the exam, know that if a route is in active state, the Feasibility Condition has failed and queries are sent.
In enterprise networks, EIGRP is often deployed in large campus or WAN environments where fast convergence is critical. The Feasibility Condition enables EIGRP to have backup routes (feasible successors) ready to use instantly. For example, a retail chain with hundreds of stores connecting to two data centers can use EIGRP with multiple WAN links. If the primary link fails, the router immediately switches to a feasible successor without any delay, avoiding application timeouts. This is possible because the Feasibility Condition ensures the backup path is loop-free.
Another scenario: A service provider uses EIGRP for MPLS VPN customer edge routers. The Feasibility Condition allows the CE router to have multiple paths to the PE routers. If one PE fails, the CE uses a feasible successor, maintaining connectivity. Network engineers tune metrics (e.g., setting bandwidth on serial links) to influence which routes become feasible successors. For instance, by increasing delay on a backup link, the AD becomes larger, potentially failing the FC. Engineers must ensure that at least one backup path meets the FC to avoid active queries.
Scale considerations: EIGRP can handle thousands of routes, but the topology table can become large. The Feasibility Condition reduces the number of routes considered for backup, but in very large networks, the number of feasible successors per route is limited (typically 1-2). Misconfiguration occurs when route redistribution causes inconsistent metrics, leading to FC failures. For example, redistributing a static route with a low metric can artificially lower the FD, making all other ADs higher, thus no feasible successors. This triggers active queries every time the successor fails, causing convergence delays. Engineers must carefully plan metric assignments.
Performance: The FC check is performed on every update, but it is computationally lightweight. The real performance consideration is the number of routes in the topology table. With default settings, EIGRP can scale to several thousand routes without issues. However, if the FC fails for many routes, DUAL queries can flood the network, impacting CPU and bandwidth. Proper network design ensures that each route has at least one feasible successor, keeping the network stable.
The CCNA 200-301 exam tests the Feasibility Condition under exam objective 3.5 (Configure and verify EIGRP). Expect scenario questions where you must identify feasible successors, determine if a route meets the FC, or predict what happens when a successor fails.
Common wrong answers: 1. "A route is a feasible successor if its AD is less than the total metric." Wrong. The comparison is AD < FD, not total metric. The total metric includes the local cost to the neighbor. 2. "If AD equals FD, the route is a feasible successor." Wrong. The condition requires strictly less. Equality fails the FC. 3. "Feasible successors are installed in the routing table." Wrong. Only the successor is in the routing table. Feasible successors are in the topology table. 4. "The Feasibility Condition is used to select the successor." Wrong. The successor is the route with the lowest metric. The FC is used to select backup routes.
Specific values and commands:
- Know the command show ip eigrp topology and how to read the output: P means passive (stable), A means active (querying). The numbers in parentheses: (total metric/AD).
- The default variance is 1, meaning only equal-cost load balancing. To use unequal-cost load balancing, variance must be configured, and routes must meet the FC.
- The FD is the lowest metric ever for that route. It can be reset with clear ip eigrp topology.
- On the exam, you may be given a topology table and asked to identify feasible successors. Look for routes where AD < FD.
Calculation traps: - If you are given metric values, ensure you distinguish between total metric and AD. The AD is the second number in parentheses. - Remember that FD is the best total metric, not the AD of the successor. The FD is the metric of the successor.
Decision rule: For scenario questions asking if a route is a feasible successor, always compare AD to the FD (the metric of the current best path). If AD < FD, yes. Otherwise, no. If the route is the successor itself, it is not a feasible successor (it's the successor).
The Feasibility Condition: AD < FD (Advertised Distance strictly less than Feasible Distance).
FD is the best metric ever computed for a destination; it is the metric of the successor route.
AD is the metric reported by a neighbor for a destination.
Feasible successors are loop-free backup routes stored in the topology table, not the routing table.
If no feasible successor exists when the successor fails, the route enters active state and DUAL queries neighbors.
Use 'show ip eigrp topology' to view FD, AD, successors, and feasible successors.
The Feasibility Condition enables fast convergence by allowing immediate use of backup routes without queries.
These come up on the exam all the time. Here's how to tell them apart.
EIGRP Feasibility Condition
Uses distance vector logic: AD < FD
Requires FD to be the best metric ever
Backup routes are precomputed and stored in topology table
Fast convergence (sub-second) without queries if FS exists
Cisco proprietary
OSPF Loop-Free Alternate (LFA)
Uses link-state database and SPF to compute backup paths
Requires the backup next-hop to be closer to destination than the router itself (downstream condition)
Backup routes are computed on-demand or precomputed depending on implementation
Fast convergence with precomputed LFAs; otherwise slower
Open standard (RFC 5286)
Mistake
A feasible successor is installed in the routing table.
Correct
Only the successor (best route) is installed in the routing table. Feasible successors are kept in the topology table as backups and are used only if the successor fails.
Candidates confuse feasible successors with the successor because both appear in the topology table output.
Mistake
The Feasibility Condition compares AD to the total metric via the neighbor.
Correct
The FC compares AD to the FD, which is the metric of the current best path (successor). The total metric via the neighbor includes the local cost and is not used in the FC.
The output shows both total metric and AD, so candidates mistakenly think the total metric is used.
Mistake
If AD equals FD, the route is a feasible successor.
Correct
The condition requires AD < FD. Equality does not guarantee loop-free operation; in fact, it may indicate a potential loop. Cisco's implementation treats equality as failing the FC.
Candidates think 'less than or equal to' is the condition, but the actual condition is strictly less.
Mistake
The Feasibility Condition is used to select the successor route.
Correct
The successor is selected based on the lowest metric. The FC is used to select feasible successors (backup routes). The successor is always the route with the smallest total metric.
Both involve route selection, but the FC is specifically for backup route validation.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
Feasible Distance (FD) is the best metric a router has ever computed for a destination. It is the metric of the successor route. Advertised Distance (AD) is the metric a neighbor reports for that destination. The FD is used as a reference to determine if a neighbor's AD is low enough to be a loop-free backup. The FD only changes when the successor changes or after a clear topology command.
No. The Feasibility Condition requires AD to be strictly less than FD. If AD equals FD, the condition fails, and the route is not considered loop-free. In practice, Cisco's EIGRP implementation treats equality as failing the condition, so the route will not be used as a feasible successor.
Look for routes that are listed under a destination but are not the successor. The successor is the first entry under the route. Any additional entries with an AD that is less than the FD (shown as 'FD is <value>') are feasible successors. For example, if FD=3072 and an entry shows AD=2816, that is a feasible successor. If AD is >= FD, it is not.
The Feasibility Condition ensures loop-free routing by preventing a router from using a path that might loop back. It guarantees that the neighbor is closer to the destination than the local router. This allows EIGRP to precompute backup routes (feasible successors) that can be used immediately without queries, enabling fast convergence.
Yes, the Feasibility Condition applies to all routes in the EIGRP topology table, including internal and external routes. However, for external routes, the administration distance and metric calculation may differ, but the FC check remains the same: AD < FD.
Clearing the topology table with 'clear ip eigrp topology' resets the Feasible Distance for all routes. The router will then re-learn routes from neighbors, and the FD will be recalculated based on the first update received. This can be useful after major topology changes or when troubleshooting FC issues.
No. Unequal-cost load balancing in EIGRP requires the variance command. However, only routes that meet the Feasibility Condition (AD < FD) can be used for unequal-cost load balancing. If a route fails the FC, it cannot be used even if variance is configured.
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