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EIGRPCCNA 200-301

EIGRP Backup Route Not Installing — Feasibility Condition Not Met

Presenting Symptom

A backup route via EIGRP is not being installed in the routing table even though the neighbor is reachable and routes are being exchanged.

Network Context

Small branch office with two routers (R1 and R2) connected via a WAN link, both running EIGRP AS 100. R1 has a primary route to a remote network 10.1.1.0/24 via a serial link with a lower metric, and a secondary path via R2 with a higher metric. The network uses EIGRP named mode with default K-values. The issue is that the backup route via R2 is not installed as a feasible successor because the reported distance from R2 is not less than the feasible distance of the successor.

Diagnostic Steps

1

Check the routing table for the missing route

show ip route 10.1.1.0
Routing entry for 10.1.1.0/24
  Known via "eigrp 100", distance 90, metric 28160, type internal
  Last update from 192.168.1.1 on Serial0/0/0, 00:00:05 ago
  Routing Descriptor Blocks:
  * 192.168.1.1, from 192.168.1.1, 00:00:05 ago, via Serial0/0/0
      Route metric is 28160, traffic share count is 1
      Total delay is 200 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1

Only one route is present (the successor). The backup route via R2 is missing. This indicates that EIGRP did not consider the alternate path as a feasible successor.

2

Examine EIGRP topology table for the destination

show ip eigrp topology 10.1.1.0/24
P 10.1.1.0/24, 1 successors, FD is 28160
         via 192.168.1.1 (28160/2560), Serial0/0/0
         via 192.168.2.2 (30720/28160), Serial0/0/1

The topology table shows two routes: one via 192.168.1.1 (successor) with FD 28160 and RD 2560, and another via 192.168.2.2 with metric 30720 and RD 28160. The backup route is not a feasible successor because the reported distance (28160) is not less than the feasible distance (28160) of the successor. The feasibility condition requires RD < FD, but here RD equals FD, so it fails.

3

Check EIGRP interfaces and neighbors

show ip eigrp neighbors
H   Address         Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                        (sec)         (ms)       Cnt Num
0   192.168.1.1     Se0/0/0         13   00:10:00   1     100  0  5
1   192.168.2.2     Se0/0/1         14   00:09:45   2     100  0  4

Both neighbors are up and exchanging routes. The problem is not a neighbor adjacency issue.

4

Verify EIGRP metrics and K-values

show ip protocols | section EIGRP
EIGRP-IPv4 Protocol for AS(100)
  Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
  NSF-aware route hold timer is 240
  Router-ID: 10.0.0.1
  Topology : 0 (base)
    Active Timer: 3 min
    Distance: internal 90 external 170
    Maximum path: 4
    Maximum hopcount 100
    Maximum metric variance 1

Default K-values are in use. The metric calculation is standard. Variance is 1, so only equal-cost load balancing is allowed. To install the backup route, either the feasibility condition must be met (RD < FD) or variance must be increased.

Root Cause

The backup route via R2 has a reported distance (RD) of 28160, which is equal to the feasible distance (FD) of the successor (28160). The EIGRP feasibility condition requires RD < FD for a route to be considered a feasible successor. Since RD equals FD, the route is not installed as a backup. This can occur when the alternate path has the same metric as the successor's RD, often due to equal bandwidth/delay on the path from the neighbor to the destination.

Resolution

To force the backup route to be installed, increase the variance value on R1 to allow unequal-cost load balancing. For example, set variance to 2 to allow routes with metric up to 2 times the FD. Commands: ``` router eigrp 100 variance 2 ``` This will install the route via R2 as a feasible successor even though the feasibility condition is not strictly met, as long as the metric is within the variance multiplier.

Verification

After applying variance, verify the route is installed: ``` show ip route 10.1.1.0 ``` Expected output should show both routes: ``` Routing entry for 10.1.1.0/24 Known via "eigrp 100", distance 90, metric 28160, type internal Last update from 192.168.1.1 on Serial0/0/0, 00:00:05 ago Routing Descriptor Blocks: * 192.168.1.1, from 192.168.1.1, 00:00:05 ago, via Serial0/0/0 Route metric is 28160, traffic share count is 1 * 192.168.2.2, from 192.168.2.2, 00:00:05 ago, via Serial0/0/1 Route metric is 30720, traffic share count is 1 ``` Also check topology table to confirm the route is now a feasible successor: ``` show ip eigrp topology 10.1.1.0/24 ``` Expected output: ``` P 10.1.1.0/24, 2 successors, FD is 28160 via 192.168.1.1 (28160/2560), Serial0/0/0 via 192.168.2.2 (30720/28160), Serial0/0/1 ```

Prevention

["Design EIGRP networks with careful metric planning to ensure backup paths have a reported distance less than the feasible distance of the successor.","Use the 'variance' command appropriately to allow unequal-cost load balancing when feasible successors are not automatically selected.","Monitor EIGRP topology tables regularly to identify routes that are not being installed due to feasibility condition failures."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests understanding of EIGRP feasibility condition and the use of variance for unequal-cost load balancing. Questions may present a troubleshooting scenario where a backup route is missing, and candidates must identify that the feasibility condition is not met and apply variance. The exam may include drag-and-drop steps to configure variance or multiple-choice questions about why a route is not in the routing table.

Exam Tips

1.

Remember the feasibility condition: RD < FD. If RD >= FD, the route is not a feasible successor.

2.

The variance command allows routes with metric up to variance * FD to be installed, even if they are not feasible successors.

3.

Know that 'show ip eigrp topology' displays both the FD and RD for each path.

Commands Used in This Scenario

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.

show ip protocols

Displays the current state of all IP routing protocols running on the router, including timers, filters, and network advertisements.

show ip route

Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.

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