EIGRP Variance

EIGRP Variance is all about unequal path load balancing.

By default, EIGRP only load balances on paths that have identical metrics and meet the feasibility condition.

A quick side note: The feasibility condition is a mechanism to ensure a loop-free path to the destination network. Essentially it states that the distance to a network, as reported by a neighbour (the reported distance), must be strictly less then the distance the router currently knows about (the feasible distance).

So what is the feasible distance? The feasible distance the smallest distance recorded to a destination network since the network last transitioned from active to passive (the state when the network is fully converged).

It’s like saying to a neighbouring router “The last time I looked I was three hops from the destination network, as long as you are closer to the destination then I’ll consider you a good bet”

Back to variance…

By default EIGRP only loads the best path (the successor) into the routing table, unless it has more than one path with an identical metric to the destination, in which case it loads up to four paths (by default) or more if the maximum-paths command  is set higher.

The above diagram shows an EIGRP network connecting 10.1.1.0/24 (PC-1) to 10.4.4.0/24 (PC-2).

The routing table on R1 shows one route to 10.4.4.0/24 via R2 (as you would expect).

R1#show ip route | i 10.4.4.0
D 10.4.4.0/24 [90/204800] via 1.2.1.2, 00:09:40, FastEthernet1/0

However, looking at the EIGRP topology table it shows 1 successor, and 1 feasible successor

R1#show ip eigrp topology
EIGRP-IPv4 VR(test) Topology Table for AS(100)/ID(10.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
<snip>
P 10.4.4.0/24, 1 successors, FD is 26214400
via 1.2.1.2 (26214400/19660800), FastEthernet1/0 < This is the successor, with a computed distance of 26214400
via 1.3.1.3 (26869760/20316160), FastEthernet1/1 < This is the feasible successor, with a computed distance of 26869760

The reason there is a feasible successor, is because the feasibility condition is met. i.e. the reported distance from 1.3.1.3 is less than the feasible distance. However only the route via 1.2.1.2 is installed in the routing table, because EIGRP does not load balance unequal paths by default. This is where the variance command comes in.

The router will make the feasible successor, a successor, if the following holds true:

The computed distance is less than feasible distance ( times the variance multiplier). So, if we were to make the variance multiplier 2, the feasible distance in this case would become 2 x 2621440 or  5242880. The feasible successor has a computed distance of 26869760, and therefore becomes a successor, and is entered into the routing table.

R1#show ip eigrp topology
EIGRP-IPv4 VR(test) Topology Table for AS(100)/ID(10.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
<snip>
P 10.4.4.0/24, 2 successors, FD is 26214400
via 1.2.1.2 (26214400/19660800), FastEthernet1/0
via 1.3.1.3 (26869760/20316160), FastEthernet1/1

R1#show ip route eigrp
<snip>
D 10.4.4.0/24 [90/209920] via 1.3.1.3, 00:21:52, FastEthernet1/1
              [90/204800] via 1.2.1.2, 00:21:52, FastEthernet1/0