The Interior Gateway Routing Protocol (IGRP) is a Cisco-proprietary routing protocol for IP. Like IP RIPv1, it is a distance vector protocol. However, it scales better than RIP because of these advantages:
• It uses a sophisticated metric based on bandwidth and delay.
• It uses triggered updates to speed-up convergence.
• It supports unequal-cost load balancing to a single destination. IGRP uses a composite metric, which includes bandwidth, delay, reliability, load, and MTU, when choosing paths to a destination. By default, the algorithm uses only bandwidth and delay.
The MTU (Maximum Transmission Unit) setting determines the largest packet size that can be transmitted through a network.
RIP (Routing Information Protocol) is a routing protocol that uses hop count as a metric to find the best path between the source and the destination networks.
Configuring IP IGRP
Router(config)# router igrp autonomous_system_#
Router(config-router)# network IP_network_#
Unlike RIP, IGRP understands the concept of an autonomous system and requires you to configure the autonomous system number in the routing process. For routers to share routing information, they must be in the same AS.
An AS (Autonomous System) is a group of networks that controls an IP address space and uses the same routing protocol and has a unified routing policy. Typically, each AS is operated by a single large organization.
With RIP, you don't need to configure anything to enable equal-cost load balancing; and RIP doesn't support unequal-cost load balancing. IGRP supports both equal- and unequal-cost paths for load balancing to a single destination. Equal-cost paths are enabled by default, where IGRP supports up to six equal-cost paths (four by default) to a single destination in the IP routing table. IGRP, however, also supports unequal-cost paths, but this feature is disabled by default.
The variance feature allows you to include equal- and unequal-cost IGRP routes in the routing table.
Router(config-router)# variance multiplier
Troubleshooting IP IGRP
show ip protocols
show ip route
debug ip igrp events
debug ip igrp transactions
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