IGRP Load Balancing

IGRP has the capability to perform multipath routing, up to four different routes to a given destination. This multipath routing enables IGRP to perform load balancing of both an equal-cost and unequal-cost nature.

IGRP Equal-Cost Load Balancing

Equal-cost load balancing is the balancing of traffic across redundant, or multiple, links of equal cost. Equal-cost load balancing may be achieved on a per-packet or a per-destination basis.

Per-packet load balancing means that sequential packets in a data stream, bound for the same destination, may be transmitted out different interfaces. The drawback to this type of operation is that data packets may be received late or out of sequence, causing possible application performance issues.

Per-destination load balancing means that packets in a data stream bound for the same destination will be forwarded via the same route, alleviating the potential for issues caused by per-packet load balancing. Per-destination load balancing can result in a less-than-ideal equal-cost traffic distribution, however.

IGRP determines which of these approaches to use based on its ability to perform route caching, a technique used to keep a route cached in memory. Route caching is per-destination-based load balancing and can be memory intensive. If route caching is disabled, per-packet load balancing will automatically be performed.

IGRP Unequal-Cost Load Balancing

Unequal-cost load balancing sends traffic across up to four paths of unequal cost, with the lowest-cost link being the primary path. Paths with higher cost are used as alternatives, providing redundant link connectivity to a single, or multiple, destination(s).

IGRP Feasibility and Feasible Successors

IGRP feasibility means that each network path conforms to three basic principles:

  • Alternative path metrics must be within the specified variance range of the local best metric.

  • The best local metric must be greater than the metric for the same destination that is learned from the next router; simply stated, the next hop must be closer to the destination than the current router.

  • The variance value, multiplied against the best local metric for a destination, must be greater than or equal to the cost metric for that destination on the next router.

IGRP can be used to establish a hierarchy of feasible successors in a multipath routing environment. Feasible successors are routes whose costs are greater than the specified variance from the optimal route to a given destination. As such, they are not feasible for unequal-cost load balancing, but are feasible routes in the event the primary route becomes unavailable.



Network Sales and Services Handbook
Network Sales and Services Handbook (Cisco Press Networking Technology)
ISBN: 1587050900
EAN: 2147483647
Year: 2005
Pages: 269

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