EIGRP or 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.
Configure Load Balancing on Cisco EIGRP using Unequal Routes
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 99. 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.
1. Examine the 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 10.5.5.5 Routing entry for 10.5.5.5/32 Known via "eigrp 99", distance 90, metric 435200, type internal Redistributing via eigrp 99 Last update from 10.12.12.2 on FastEthernet0/0, 00:01:30 ago Routing Descriptor Blocks: * 10.12.12.2, from 10.12.12.2, 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 10.12.12.2, which means packet will go through link A and R2 is the best next-hop from R1 to reach R5. This is reasonable because as we can see on the topology above, link A/D/E is a fastethernet connection and link B/C/F is a serial connection. We know that fastethernet is more superior than serial connection and has lower metric value. The path through R2 consists of link A and link D, which are all 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.
2. Identify EIGRP Feasible Successor (FS) routes
Feasible Successor or FS route is the next route that will be installed to the routing table in case the current best route is broken. To see if there any FS route to our destination, use command show ip eigrp topology | section [destination IP]
R1#show ip eigrp topology | section 10.5.5.5 P 10.5.5.5/32, 1 successors, FD is 435200 via 10.12.12.2 (435200/409600), FastEthernet0/0 via 10.13.13.3 (2323456/409600), Serial0/0
The output above shows two routes from R1 to reach R5. The first one and also the Successor route is via 10.12.12.2 or R2. 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 or called the Feasible Distance (FD) is 435200 and advertised distance of R5 from R2 is 409600.
The second one is via 10.13.13.3 or R3 that is installed as the FS 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 which will be shown in step 3 later.
3. Configure the variance value
Variance value is a multiplier that applied to the FD, and if the multiplication result is bigger than the FS metric that FS 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 99 R1(config-router)#variance 6
In this scenario, using variance value less than 6 will not install the FS 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 FS metric which is 2322456 as confirmed in step 2. Configuring it to more than 6 is also inefficient as there is only one available FS 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 10.5.5.5 Routing entry for 10.5.5.5/32 Known via "eigrp 99", distance 90, metric 435200, type internal Redistributing via eigrp 99 Last update from 10.13.13.3 on Serial0/0, 00:00:13 ago Routing Descriptor Blocks: 10.13.13.3, from 10.13.13.3, 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 * 10.12.12.2, from 10.12.12.2, 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 10.12.12.2 (R2) and 10.13.13.3 (R3). These two routes will be used in load balance even though it has unequal metric.
Variance value can be used to configure load balancing on Cisco EIGRP using several routes with unequal metric value. Note that it requires the destination route to have an FS route. And it is also should be noted that variance value has to be configured with the right value so that it makes the multiplication with the FD can resulting in bigger value than the FS metric. However, variance configuration takes effect on the whole EIGRP routing table, therefore administrator has to consider the other impact it may cause to the other destinations.
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