Unequal Cost Load Balancing (UCLB)

ginglyformweekNetworking and Communications

Oct 29, 2013 (3 years and 5 months ago)


Unequal Cost Load Balancing (UCLB)

If there are multiple paths to the same destination with equal costs, it is reasonable to use
them all and share traffic equally among the paths. Alternate paths are guaranteed to be
free, as they are “symmetric”
with respect to cost to the primary path. If we there are
multiple paths of unequal cost, the same idea could not be applied easily.

In unequal cost load balancing, amount of traffic send across a particular path is inversely
proportional to the path’s met

Calculate Variance for the network given below:

If you are using only delay to calculate the metric, what should be the value of variance to
allow unequal cost load balancing to reach R2’s loopback address from R1, if EIGRP is
configured as the ro
uting protocol?

To calculate variance use the formula given below:

metric < best_metric*variance


is the full metric of the alternate path and


is the metric of the primary path.

Normally variance value is 1, meaning that only equal
ost paths are used

The best way to implement UCLB in real life is by using MPLS Traffic Engineering. This
solution allows for high level of administrative control, and is IGP agnostic (well they say you
need OSPF or ISIS, but you may use verbatim MPLS TE
tunnels even with RIP or EIGRP). It is
safe to use unequal
cost load
balancing with MPLS TE tunnels because they connect two
nodes using “virtual circuits” and no transit node ever performs routing lookup