A composite metric in a classful protocol
IGRP is a legacy Cisco distance-vector protocol for IPv4. The route advertisement omits the subnet mask, as in RIPv1. It uses a composite metric that by default favors higher minimum bandwidth along the path and lower cumulative configured delay.
The bottleneck bandwidth is the slowest link in the path. Delay accumulates across links. Load and reliability can be weighted as well. Two paths with the same hop count can therefore have different IGRP metrics.
Periodic broadcasts and route aging
IGRP periodically broadcasts routing information on participating interfaces, with a default interval of 90 seconds. A receiver ages routes when no refresh arrives. When a path fails, the protocol can advertise the loss and use a hold-down interval to limit the effect of stale information.
IGRP predates EIGRP. Use a legacy lab image with explicit IGRP support to observe its behavior.
Try it in a lab
- On IGRP-capable lab routers, advertise a destination through two paths with equal bottleneck bandwidth but different total configured delays. Use consistent classful subnetting and the same IGRP process number across neighbors.
- Compare the two metrics and verify that the lower-delay path is preferred with default metric weights.
- Change the configured delay so the other path has the lower cumulative value, then allow the updates to propagate and verify the next hop changes.