Technology: Routing
Area: IGP
Vendor: Cisco
Software: 12.X , 15.X, IP Base, IP Services
Platform: Catalyst 3560, 3750, 3850, 4500, 6500, ISR Routers

Enhanced Interior Gateway Routing Protocol is a Cisco proprietary technology for routing. It calculates the best route to a destination using metric value that is based on the link bandwidth and delay. When there are several routes with an equal metric value, by default all dynamic routing protocols, including EIGRP, will used all these routes to forward packet in load balance style. However, there is a way to configure load balancing on Cisco EIGRP using routes with different metric values. This unique feature on EIGRP is called unequal cost load balancing and it’s very easy to be implemented.

Unequal Routes Load Balancing on Cisco EIGRP  :
In this scenario, the topology and IP addressing is as shown in figure below and the routing protocol has been configured using EIGRP with AS number 100. Administrator has to ensure that communication between R1 and R5 loopback address will be sent in load balance using at least two routes. Before we start the analysis and configuration, it is best to review the basic EIGRP terminology because we are going to be dependent on it. 

 

• Verifying current best route to the destination :

On the originating router, use command "show ip route [destination IP]" to show the best route to reach the destination IP. In this scenario, the output on R1 will be like this:

R1# show ip route 5.5.5.5
Routing entry for 5.5.5.5/32
Known via "eigrp 100", distance 90, metric 435200, type internal
Redistributing via eigrp 100
Last update from 20.12.12.2 on FastEthernet0/0, 00:01:30 ago
Routing Descriptor Blocks:
* 20.12.12.2, from 20.12.12.2, 00:01:30 ago, via FastEthernet0/0
Route metric is 435200, traffic share count is 1
Total delay is 7000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2

We can see the next-hop address is 20.12.12.2, which means packet will go through link A and R2 is the best next-hop from R1 to reach R5. This is reasonable due to connection type between the routers. We know that fastethernet is more superior than serial connection and has lower metric value. The path through R2 is  using fastethernet connection. Compared to the other two paths in this topology (through R3 and R4) which includes link with serial connection, the path through R2 is obviously the one with lowest metric value, and that’s why we’re seeing it installed in the routing table

•  Verify EIGRP Feasible Successor (FS) routes :

Feasible Successor is the backup route that will be installed to the routing table in case the current best route is broken. To see if there any Feasible Successor route to our destination, use command "show ip eigrp topology | sec [destination IP]"

R1# show ip eigrp topology | sec 5.5.5.5
P 5.5.5.5/32, 1 successors, FD is 435200
via 20.12.12.2 (435200/409600), FastEthernet0/0
via 20.13.13.3 (2323456/409600), Serial0/0

 

 We can see the output above shows two routes from R1 to reach R5. The first one and also the Successor route is via 20.12.12.2 (R20).

Successor route is a route that has been chosen as the best route and installed in the routing table, and in this scenario we have confirmed in step 1 that route through R2 is installed in the routing table. From the output above, the metric value of the successor route is 435200 and advertised distance of R5 from R2 is 409600.

The second one is via 20.13.13.3 (R3) that is installed as the Feasible Successor route with metric value 2322456 and advertised distance of R5 from R3 is 409600. The route through R3 meets the feasibility requirement because the advertised distance (409600) is smaller than the metric of the FD (metric via R2 is 435200).

This FS route can be used to configure load balancing on Cisco EIGRP.

 

• Configure the EIGRP Load-balance using variance value :

Variance value is a multiplier that applied to the Successor, and if the multiplication result is bigger than the Feasible Successor metric, that Feasible Successor route will also be installed to the routing table along with the best route for load balance.

Variance value is configured under EIGRP routing configuration with command "variance [value]". The default variance value is 1 and maximum is 128. Below is the configuration for this scenario where the variance is given the value of 6.

R1(config)#router eigrp 100
R1(config-router)#variance 6

 

using variance value less than 6 will not install the Feasible Successor into routing table. Because the current best metric is 435200 and multiplying it by any value less than 6 will not make it bigger than the Feasible Successor metric which is 2322456 as confirmed in the Feasible successor route verification above.

Configuring it to more than 6 is also inefficient as there is only one available Feasible Successor for route to R5. To confirm the routing table is updated after applying variance, use the same command as shown in step 1

R1(config-router)# do show ip route 5.5.5.5
Routing entry for 5.5.5.5/32
Known via "eigrp 100", distance 90, metric 435200, type internal
Redistributing via eigrp 100
Last update from 20.13.13.3 on Serial0/0, 00:00:13 ago
Routing Descriptor Blocks:
20.13.13.3, from 20.13.13.3, 00:00:13 ago, via Serial0/0
Route metric is 2323456, traffic share count is 3
Total delay is 26000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2
* 20.12.12.2, from 20.12.12.2, 00:00:13 ago, via FastEthernet0/0
Route metric is 435200, traffic share count is 16
Total delay is 7000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2

From the output above, we can see that there are two routes installed now which are via 20.12.12.2 (R2) and 20.13.13.3 (R3). These two routes will be used in load balance even though it has unequal metric.