Courseiva
Exam ScenariosCCNA 200-301

EIGRP Feasibility Condition Calculation on Exam

Presenting Symptom

A router in the EIGRP domain is not installing a feasible successor route for a network, even though an alternate path exists with a reported distance less than the feasible distance of the successor.

Network Context

The network consists of three routers (R1, R2, R3) in a small branch office running EIGRP with AS 100. R1 is the headquarters router, R2 and R3 are branch routers. All routers run IOS 15.x. The topology is a triangle: R1 connects to R2 and R3, and R2 connects to R3. The network 10.1.1.0/24 is advertised from R1. R2 learns this network via R1 (successor) and via R3 (alternate path). The alternate path via R3 has a reported distance of 1280, but the feasible distance on R2 is 1280 as well, causing the feasibility condition to fail.

Diagnostic Steps

1

Check EIGRP topology table for the prefix

show ip eigrp topology 10.1.1.0/24
P 10.1.1.0/24, 1 successors, FD is 1280
        via 10.1.12.1 (1280/512), Serial0/0/0
        via 10.1.23.3 (1536/1280), Serial0/0/1

The output shows the successor via R1 with FD 1280 and RD 512. The alternate path via R3 has RD 1280, which is equal to the FD (1280). For the feasibility condition, the RD must be strictly less than the FD. Since RD equals FD, the condition fails, and the route is not installed as a feasible successor.

2

Verify EIGRP neighbor relationships

show ip eigrp neighbors
H   Address         Interface   Hold Uptime   SRTT   RTO  Q  Seq
                                   (sec)         (ms)       Cnt Num
0   10.1.12.1       Se0/0/0      13   00:12:34   1   200  0  12
1   10.1.23.3       Se0/0/1      14   00:10:20   2   200  0  15

Both neighbors are up and stable. This rules out neighbor issues. The problem is not with adjacency but with the metric calculation.

3

Check the metric components on the alternate path

show interface serial0/0/1
Serial0/0/1 is up, line protocol is up
  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,
     reliability 255/255, txload 1/255, rxload 1/255

The interface bandwidth is 1544 Kbps and delay is 20000 microseconds. These values are used to compute the metric. The reported distance from R3 includes the metric from R3 to the destination. If the bandwidth or delay on R3's interface to R1 is lower, it could cause a higher RD. However, the issue here is that the RD equals the FD, not that it is higher.

4

Check the metric on R3 for the same prefix

show ip eigrp topology 10.1.1.0/24 on R3
P 10.1.1.0/24, 1 successors, FD is 1280
        via 10.1.13.1 (1280/512), Serial0/0/0

On R3, the FD is also 1280. This means the metric from R3 to the destination is 1280. When R3 advertises this route to R2, it includes its FD (1280) as the RD. Since R2's FD is also 1280, the feasibility condition fails. The root cause is that the metrics are identical, preventing the alternate path from being a feasible successor.

Root Cause

The alternate path via R3 has a reported distance (RD) of 1280, which is equal to the feasible distance (FD) of 1280 on R2. EIGRP's feasibility condition requires the RD to be strictly less than the FD. Since they are equal, the route is not installed as a feasible successor, and the network lacks a backup path.

Resolution

To fix this, adjust the EIGRP metric on one of the links to make the RD strictly less than the FD. For example, increase the delay on the link between R1 and R2 to increase the FD on R2, or decrease the delay on the link between R3 and R1 to reduce the RD advertised by R3. Here, we will increase the delay on R1's interface to R2: On R1: interface Serial0/0/0 delay 20000 end This increases the delay from 20000 to 20000 microseconds (actually, we need to increase it further; let's set delay to 30000). Alternatively, on R3, decrease the delay on the interface to R1: On R3: interface Serial0/0/0 delay 10000 end This reduces the metric on R3, making its RD lower. After the change, the FD on R2 remains 1280, but the RD from R3 becomes lower (e.g., 1024), satisfying the feasibility condition.

Verification

After applying the fix, run on R2: show ip eigrp topology 10.1.1.0/24 Expected output: P 10.1.1.0/24, 1 successors, FD is 1280 via 10.1.12.1 (1280/512), Serial0/0/0 via 10.1.23.3 (1536/1024), Serial0/0/1 The RD from R3 is now 1024, which is less than the FD of 1280, so the route is now a feasible successor. Also verify with: show ip route 10.1.1.0 Expected output shows only the successor route, but the feasible successor is in the topology table.

Prevention

["Design EIGRP metrics with unequal costs to ensure feasibility condition can be met for backup paths.","Use offset-lists or delay adjustments to create metric diversity when multiple paths have similar metrics.","Monitor EIGRP topology table regularly to ensure feasible successors exist for critical prefixes."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a candidate must identify why a feasible successor is not installed. The exam tests understanding of the feasibility condition: the reported distance must be less than the feasible distance. Candidates may be asked to interpret show ip eigrp topology output and determine if a route qualifies as a feasible successor. Multiple-choice questions often present topology table output and ask why a particular path is not used.

Exam Tips

1.

Memorize the feasibility condition: RD < FD for a route to be a feasible successor.

2.

In show ip eigrp topology output, the first number in parentheses is the composite metric (FD), the second is the RD. Compare the RD of the alternate path to the FD of the successor.

3.

If RD equals FD, the route is not a feasible successor. The exam may trick you by showing equal values and asking if the route is a feasible successor.

Commands Used in This Scenario

show interfaces

Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.

show ip eigrp neighbors

Displays the neighbor table for EIGRP, showing all directly connected EIGRP routers and their status, used to verify EIGRP adjacencies and troubleshoot neighbor relationships.

show ip eigrp topology

Displays the EIGRP topology table, showing all learned routes and their feasible successors, used to verify EIGRP convergence and path selection.

Test Your CCNA Knowledge

Practice with scenario-based questions to prepare for the CCNA 200-301 exam.

Practice CCNA Questions