EIGRP Variance

Lessons Discussion
1

Hello

I enabled the variance on R-0 and see unequal cost load balancing on the routes below:

P 192.168.110.0/24, 2 successors, FD is 131072
via 192.168.20.2 (131072/130816), GigabitEthernet0/0
via 192.168.10.1 (2297856/128256), Serial0/3/0

BUT I CANNOT SEE unequal cost load balancing on the routes below!

P 192.168.12.0/24, 1 successors, FD is 3072
via 192.168.20.2 (3072/2816), GigabitEthernet0/0
via 192.168.10.1 (2170112/2816), Serial0/3/0

R-0#sho ip eigrp topology
IP-EIGRP Topology Table for AS 123

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status

P 192.168.20.0/24, 1 successors, FD is 2816
via Connected, GigabitEthernet0/0
P 192.168.10.0/24, 1 successors, FD is 2169856
via Connected, Serial0/3/0
P 192.168.100.0/24, 1 successors, FD is 128256
via Connected, Loopback100
P 192.168.110.0/24, 2 successors, FD is 131072
via 192.168.20.2 (131072/130816), GigabitEthernet0/0
via 192.168.10.1 (2297856/128256), Serial0/3/0
P 192.168.12.0/24, 1 successors, FD is 3072
via 192.168.20.2 (3072/2816), GigabitEthernet0/0
via 192.168.10.1 (2170112/2816), Serial0/3/0
P 192.168.120.0/24, 1 successors, FD is 130816
via 192.168.20.2 (130816/128256), GigabitEthernet0/0

R-0#sho ip route eigrp
192.168.10.0/24 is variably subnetted, 2 subnets, 2 masks
D 192.168.12.0/24 [90/3072] via 192.168.20.2, 00:01:48, GigabitEthernet0/0
192.168.100.0/24 is variably subnetted, 2 subnets, 2 masks
D 192.168.110.0/24 [90/131072] via 192.168.20.2, 00:01:48, GigabitEthernet0/0
[90/2297856] via 192.168.10.1, 00:01:49, Serial0/3/0
D 192.168.120.0/24 [90/130816] via 192.168.20.2, 00:01:48, GigabitEthernet0/0

Please help me!

Thanks
Ahmed

2

On R-0, please give us the output of the following command:

show ip eigrp topology all-links

3

R-0#sho ip eigrp topology all-links
IP-EIGRP Topology Table for AS 123

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - Reply status

P 192.168.20.0/24, 1 successors, FD is 2816
via Connected, GigabitEthernet0/0
P 192.168.10.0/24, 1 successors, FD is 2169856
via Connected, Serial0/3/0
P 192.168.100.0/24, 1 successors, FD is 128256
via Connected, Loopback100
P 192.168.110.0/24, 2 successors, FD is 131072
via 192.168.20.2 (131072/130816), GigabitEthernet0/0
via 192.168.10.1 (2297856/128256), Serial0/3/0
P 192.168.12.0/24, 1 successors, FD is 3072
via 192.168.20.2 (3072/2816), GigabitEthernet0/0
via 192.168.10.1 (2170112/2816), Serial0/3/0
P 192.168.120.0/24, 1 successors, FD is 130816
via 192.168.20.2 (130816/128256), GigabitEthernet0/0

4

Ahmed,
Your network, as configured, will be unable to do unequal load balancing. Here’s why …

Remember, that the variance command is NOT telling EIGRP the number of maximum paths (I used to think that, long ago). Instead, it is a multiplier that is used to determine whether a path should be used. Let’s break it down in your specific case.

For your route 192.168.12.0/24, we need to define our terms:

P 192.168.12.0/24, 1 successors, FD is 3072
via 192.168.20.2 (3072/2816), GigabitEthernet0/0
via 192.168.10.1 (2170112/2816), Serial0/3/0

Your Successor route is via 192.168.20.2
The Feasible Distance of your Successor route is 3072

P 192.168.12.0/24, 1 successors, FD is 3072
via 192.168.20.2 (3072/2816), GigabitEthernet0/0
via 192.168.10.1 (2170112/2816), Serial0/3/0

Your Feasible Successor route (the proposed “backup” route) is via 192.168.10.1
The Feasible Distance of your Feasible Successor route is 2170112

In order for a route to be used as part of unequal load cost balancing, the Feasible Distance of the Feasible successor has to be less than or equal to the Feasible Distance of the Successor route multiplied by the variance. So in your case, these are the numbers:

2170112 <= ( 3072 * Variance )

Ok, now, simple algebra. What is the Variance (rounded up to a whole number)? 707. This means you would have to set your variance to 707 to get that second route to load balance, but this is impossible, because the maximum allowed value of Variance is 128. This is why you only have (and will only have) one actively used route.

5

Thanks Andrew

But 2170112 is greater and not less than 3072. Because 2170112 is the backup routes?

Hos is that possible?

P 192.168.12.0/24, 1 successors, FD is 3072
via 192.168.20.2 (3072/2816), GigabitEthernet0/0
via 192.168.10.1 (2170112/2816), Serial0/3/0

Your Successor route is via 192.168.20.2
The Feasible Distance of your Successor route is 3072

P 192.168.12.0/24, 1 successors, FD is 3072
via 192.168.20.2 (3072/2816), GigabitEthernet0/0
via 192.168.10.1 (2170112/2816), Serial0/3/0

6

Yes, 2170112 is greater than 3072 * Variance. This means that 2170112 fails the check. The rule states that 2170112 has to be less than or equal to 3072 * Variance in order for it to be used as an unequal load balanced path.

Since it is impossible to select a Variance large enough to make the condition true, that route will never be used.

7

A post was merged into an existing topic: EIGRP Variance Command Example