Cisco 3560G 48TS Software Configuration Guide - Page 633

Chapter 30 Configuring IP Unicast Routing Configuring IGRP Table 30-5 Default IGRP Configuration (continued) Feature Timers basic Traffic-share Default Setting Update: 90 seconds. Invalid: 270 seconds. Hold-down: 280 seconds. Flush: 630 seconds. Sleeptime: 0 milliseconds. Distributed proportionately to the ratios of the metrics. Routers running IGRP use flash and poison-reverse updates to speed up the convergence of the routing algorithm. Flash updates are updates sent before the standard interval, notifying other routers of a metric change. Poison-reverse updates are intended to defeat larger routing loops caused by increases in routing metrics. The poison-reverse updates are sent to remove a route and place it in hold-down, which keeps new routing information from being used for a certain period of time. Understanding Load Balancing and Traffic Distribution Control IGRP can simultaneously use an asymmetric set of paths for a given destination. This unequal-cost load balancing allows traffic to be distributed among up to four unequal-cost paths to provide greater overall throughput and reliability. Alternate path variance (that is, the difference in desirability between the primary and alternate paths) determines the feasibility of a potential route. An alternate route is feasible if the next router in the path is closer to the destination (has a lower metric value) than the router being used, and if the metric for the entire alternate path is within the variance. Only feasible paths are used for load balancing and are included in the routing table. These conditions limit the number of load balancing occurrences, but ensure that the dynamics of the network remain stable. These general rules apply to IGRP unequal-cost load balancing: • IGRP accepts up to four paths for a given destination network. • The local best metric must be greater than the metric learned from the next router; that is, the next hop router must be closer (have a smaller metric value) to the destination than the local best metric. • The alternative path metric must be within the specified variance of the local best metric. The multiplier times the local best metric for the destination must be greater than or equal to the metric through the next router. If these conditions are met, the route is determined to be feasible and can be added to the routing table. By default, the amount of variance is set to one (equal-cost load balancing). Use the variance router configuration command to define how much worse an alternate path can be before that path is disallowed. If variance is configured as described in the preceding section, IGRP or Enhanced IGRP distributes traffic among multiple routes of unequal cost to the same destination. If you want faster convergence to alternate routes, but you do not want to send traffic across inferior routes in the normal case, you might prefer to have no traffic flow along routes with higher metrics. Use the traffic-share router configuration command to control distribution of traffic among multiple routes of unequal cost. 78-16156-01 Catalyst 3560 Switch Software Configuration Guide 30-25