Back
Lesson 4 of 5

Unequal-Cost Load Balancing

Lab Objectives

  • Configure EIGRP (ASN 100) on the routers in the provided topology and advertise the 192.168.3.0/24 loopback on two routers so there are multiple EIGRP-learned paths to the same prefix.
  • Configure variance to permit unequal-cost EIGRP load balancing and demonstrate how the feasibility condition determines which alternative routes can be installed in the routing table.
  • Verify topology and routing behavior with EIGRP show commands and interpret the output to understand why some candidate routes are or are not used.

Topology (use this EXACT diagram — IPs on each interface are shown)

                [Internet]
               203.0.113.1
                    |
               R1 (Gateway)
              Gi0/0: 10.10.10.1
              Gi0/1: 10.10.20.1
              Gi0/2: 10.10.30.1
              /     |     \
           R2      R3      R4

Gi0/0: 10.10.10.2 | Gi0/0: 10.10.30.2 Gi0/1: 10.10.40.1 | / \ | S1 S2 S3 / \ | /
PC1 PC2 PC3 PC4 PC5

Tip: In this lesson we will create a loopback interface on R2 and R4 both advertising the same prefix (192.168.3.0/24). That gives R1 two EIGRP-learned routes to the same prefix so we can illustrate unequal-cost load sharing using variance and show how the feasibility condition controls which alternate routes are valid.

Lab Tasks (Try It Yourself First!)

Complete these tasks WITHOUT looking at the solution below. Use ? and show commands to figure it out.

Task 1: Configure EIGRP and originate the destination network

On R2 and R4 configure a loopback interface with address 192.168.3.1/24 and enable EIGRP (ASN 100) on R1, R2, and R4 so the 10.10.10.0/24, 10.10.20.0/24, 10.10.30.0/24 and 192.168.3.0/24 networks are advertised.

Task 2: Create unequal path metrics

Adjust the interface bandwidth on R1 so that the path via Gi0/2 (to R4) has a lower EIGRP metric than the path via Gi0/0 (to R2). Use the bandwidth interface command to influence EIGRP’s metric calculation.

Task 3: Configure variance and verify unequal-cost load sharing

On R1 set variance 2 under EIGRP and verify that both routes to 192.168.3.0/24 are installed in the routing table (if they meet the feasibility condition). Use show ip eigrp topology to inspect reported distance (RD) and feasible distance (FD) values and explain why each route is or is not used.

Think About It: If you set variance 10 but the second route still does not appear in the routing table, what does that tell you about the feasibility condition for that route?

Lab Solution

Task 1 Solution: Configure EIGRP and originate the destination network

What we are doing: We will create a loopback on R2 and R4 for 192.168.3.0/24 so the same prefix originates at two routers. We then enable EIGRP AS 100 on R1, R2, and R4 and advertise the relevant networks. This ensures R1 learns two routes for the same prefix — necessary for any load-sharing decision.

! On R2
interface Loopback0
 ip address 192.168.3.1 255.255.255.0

router eigrp 100
 network 10.10.10.0 0.0.0.255
 network 10.10.40.0 0.0.0.255
 network 192.168.3.0 0.0.0.255
 no auto-summary

! On R4
interface Loopback0
 ip address 192.168.3.1 255.255.255.0

router eigrp 100
 network 10.10.30.0 0.0.0.255
 network 192.168.3.0 0.0.0.255
 no auto-summary

! On R1
router eigrp 100
 network 10.10.10.0 0.0.0.255
 network 10.10.20.0 0.0.0.255
 network 10.10.30.0 0.0.0.255
 no auto-summary

What just happened:

  • Creating the loopback on R2 and R4 causes both routers to originate the 192.168.3.0/24 network into EIGRP.
  • Enabling EIGRP with router eigrp 100 and the network statements advertises connected interfaces into EIGRP for adjacency and route exchange.
  • no auto-summary avoids classful summarization, ensuring the /24 prefixes are propagated precisely.

Verify:

! On R1: show learned EIGRP routes
show ip route eigrp

R1# show ip route eigrp
Codes: D - EIGRP, (other codes omitted)
D 192.168.3.0/24 [90/28160] via 10.10.30.2, 00:00:12, GigabitEthernet0/2

Expected: R1 should show at least one EIGRP route to 192.168.3.0/24 (the best path). At this stage you will likely see R1 preferring the path via the neighbor with the lower composite metric.

Task 2 Solution: Create unequal path metrics

What we are doing: We influence EIGRP metric calculation by changing the interface bandwidth values on R1 so that the composite metric for the path via Gi0/2 is lower than the path via Gi0/0. This creates an unequal-cost situation where one path is preferred but the other remains a candidate.

! On R1 — make Gi0/2 appear "faster" to EIGRP than Gi0/0
interface GigabitEthernet0/0
 bandwidth 1000
!
interface GigabitEthernet0/2
 bandwidth 100000

What just happened:

  • The bandwidth command adjusts the interface bandwidth value used by EIGRP in its metric formula (EIGRP uses the minimum bandwidth along the path plus delay in default metrics).
  • By setting Gi0/2 to a much higher bandwidth and Gi0/0 to a low bandwidth, the path via Gi0/2 will have a lower overall EIGRP metric, making it the preferred (successor) route at R1.

Verify:

! On R1: view EIGRP topology for the prefix to see FD and RD values
show ip eigrp topology 192.168.3.0

R1# show ip eigrp topology 192.168.3.0
IP-EIGRP Topology Table for AS(100)/ID(10.10.30.1)
Codes: P - Passive, A - Active, (other codes omitted)

P 192.168.3.0/24, 1 successors, FD is 28160
        via 10.10.30.2 (28160/28160), GigabitEthernet0/2
        via 10.10.10.2 (56320/28160), GigabitEthernet0/0 (not feasible)

Explanation of fields:

  • FD (Feasible Distance) is the best-known distance to the destination.
  • Each "via" line shows (reported distance / total metric) as (RD/FD_at_neighbor). If RD < local FD, that route is a feasible successor; otherwise it's not feasible.

Task 3 Solution: Configure variance and verify unequal-cost load sharing

What we are doing: On R1 we set variance 2. That allows EIGRP to install alternative routes whose metrics are <= (variance × FD) into the routing table — but only if they meet the feasibility condition (their reported distance is less than the current FD). We then inspect the routing table to see multiple installed EIGRP routes.

! On R1
router eigrp 100
 variance 2

What just happened:

  • variance 2 tells EIGRP to consider routes with a metric up to 2× the successor FD for installation — but only feasible successors (those satisfying RD < FD). This enables unequal-cost load sharing when multiple feasible routes exist.

Verify:

! Check routing table on R1
show ip route 192.168.3.0

R1# show ip route 192.168.3.0
Routing entry for 192.168.3.0/24
  Known via "eigrp 100", distance 90, metric 28160, type internal
  Advertised by 10.10.30.2
  Routed via GigabitEthernet0/2, 00:01:25, weight 1
  Routing Descriptor Blocks:
  * 10.10.30.2 (28160/28160), from 10.10.30.2, GigabitEthernet0/2
    10.10.10.2 (56320/28160), from 10.10.10.2, GigabitEthernet0/0

! Show EIGRP topology entries again to confirm feasibility flags
show ip eigrp topology 192.168.3.0

R1# show ip eigrp topology 192.168.3.0
P 192.168.3.0/24, 1 successors, FD is 28160
        via 10.10.30.2 (28160/28160), GigabitEthernet0/2
        via 10.10.10.2 (56320/28160), GigabitEthernet0/0 (not feasible)
  • Interpretation: Although the path via 10.10.10.2 has a total metric (56320) that is within 2× FD (2×28160 = 56320), it still cannot be installed because it fails the feasibility condition — the reported distance (RD) from the neighbor must be < FD. The topology output shows the second route is marked “not feasible” (because RD ≥ FD), so it is not a feasible successor and will not be placed in the RIB even though variance would allow the metric.

Important: EIGRP will only install alternate routes that both (a) have metric ≤ variance * FD and (b) are feasible successors (RD < FD). If a route fails the feasibility test, increasing variance will not cause it to be used.

Troubleshooting Scenario

Scenario: Second path does not appear in R1's routing table even after variance 5

Symptom: show ip route on R1 shows only one EIGRP path to 192.168.3.0/24, even though show ip eigrp topology shows another path with metric less than the variance threshold.

Your task: Find and fix the issue.

Hint: Check the reported distance values in the EIGRP topology table — feasibility depends on RD, not only on FD or variance.

Solution:

  • Use show ip eigrp topology 192.168.3.0 and compare the reported distance (RD) value for the alternate path with the local FD. If RD >= FD, the route is not a feasible successor and cannot be installed. To fix:
    • Ensure the neighbor providing the alternate route advertises a correct (and preferably smaller) RD. Often RD is large because the neighbor uses a high outbound delay or low bandwidth on its path; fix by adjusting bandwidth/delay on interfaces along that neighbor’s path (on the neighbor itself) so the neighbor reports a lower RD.
    • After altering bandwidth/delay on the neighbor interfaces, verify that RD < FD. If so, the route can become a feasible successor and, if it also meets the variance limit, will be installed.

Verification Checklist

  • EIGRP (ASN 100) configured on R1, R2, and R4 with the correct network statements.
  • Loopback0 on R2 and R4 configured with 192.168.3.1/24 and advertised into EIGRP.
  • Interface bandwidth adjusted on R1 so metrics differ between Gi0/0 and Gi0/2.
  • variance 2 set on R1 and show ip eigrp topology inspected to confirm feasibility flags.
  • Routing table on R1 shows multiple EIGRP routes only for feasible successors that meet variance.

Common Mistakes

SymptomCauseFix
Alternate route not installed despite variance setThe alternate route is not a feasible successor (reported distance >= local FD)Inspect show ip eigrp topology; adjust bandwidth/delay on the advertising neighbor so RD < FD or change path design
No EIGRP neighbors formWrong AS number or missing network statementsConfirm router eigrp 100 on all routers and correct network statements; use show ip eigrp neighbors
Two routers originate same prefix but R1 shows only one route and it flapsDuplicate networks with same prefix and misconfigured metrics or timersVerify loopbacks are stable, ensure correct interface bandwidth/delay, and check for route feedback loops

Challenge Task

Configure R3 so it participates in EIGRP and redistributes an external route (e.g., a static route to 203.0.113.1) into EIGRP as an external route. Then re-run the topology so R1 has three different path types to 192.168.3.0/24 (internal via R4, internal via R2, and an external path via R3). Your goal: show how EIGRP treats internal vs external routes during selection and explain whether variance can be applied to external EIGRP routes.

No step-by-step guidance — apply knowledge of EIGRP internal vs external route selection and how variance applies only to internal EIGRP successors.

Practice with Hands-On Labs

30 step-by-step labs for CCNA, CCNP Enterprise & Security