Thursday, June 7, 2012

EIGRP Unequal-Cost Load Balancing

Router 1's routing table has two routes  to 2.2.2.2 , 3.3.3.3 and 192.168.10.0 but I can only see the successor routes from the routing table. Network 192.168.10.0 has two successor routes because they are being equally load balanced, their AD and FD are the same [ 90/2172416] EIGRP by default will equal cost load balance 4 links but can do up 16 links.

R1#show ip route eigrp 
     2.0.0.0/32 is subnetted, 1 subnets
D       2.2.2.2 [90/2297856] via 172.12.123.2, 00:00:18, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
D       3.3.3.3 [90/2297856] via 172.12.123.3, 00:00:18, Serial0/0
D    192.168.10.0/24 [90/2172416] via 172.12.123.2, 00:00:18, Serial0/0
                     [90/2172416] via 172.12.123.3, 00:00:18, Serial0/0


Looking at the EIGRP topology table I can see all the routes


R1#show ip eigrp topology 
IP-EIGRP Topology Table for AS(100)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status 

P 3.3.3.3/32, 1 successors, FD is 2297856
        via 172.12.123.3 (2297856/128256), Serial0/0
        via 172.12.123.2 (2300416/156160), Serial0/0
P 2.2.2.2/32, 1 successors, FD is 2297856
        via 172.12.123.2 (2297856/128256), Serial0/0
        via 172.12.123.3 (2300416/156160), Serial0/0
P 1.1.1.1/32, 1 successors, FD is 128256
        via Connected, Loopback0
P 192.168.10.0/24, 2 successors, FD is 2172416
        via 172.12.123.2 (2172416/28160), Serial0/0
        via 172.12.123.3 (2172416/28160), Serial0/0
P 172.12.123.0/24, 1 successors, FD is 2169856
        via Connected, Serial0/0


Networks 2.2.2.2 and 3.3.3.3 have feasible successors routes that are pretty close to the successor routes in terms of cost, so I would like to load balance between unequal cost. Using the variance command I can set the variance between the successor metric to twice FD. Now the routing table has been updated with the load balancing routes. 


R1(config)#router eigrp 100
R1(config-router)#variance 2

R1(config)#do show ip route eigrp
     2.0.0.0/32 is subnetted, 1 subnets
D       2.2.2.2 [90/2297856] via 172.12.123.2, 00:00:15, Serial0/0
                [90/2300416] via 172.12.123.3, 00:00:15, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
D       3.3.3.3 [90/2297856] via 172.12.123.3, 00:00:15, Serial0/0
                [90/2300416] via 172.12.123.2, 00:00:15, Serial0/0
D    192.168.10.0/24 [90/2172416] via 172.12.123.2, 00:00:15, Serial0/0
                     [90/2172416] via 172.12.123.3, 00:00:15, Serial0/0

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