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.
Step-by-step:
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
Explanation:
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.
