• Holddowns
• Split horizons
• Poison reverse updates

Holddowns
Holddowns are used to prevent regular update messages from inappropriately reinstating a route that may not be up. When a router goes down, neighboring routers detect this via the lack of regularly scheduled update messages.

Split horizons
Split horizons are derived from the premise that it is usually not useful to send information about a route back in the direction from which it came. The split horizon rule helps prevent routing loops.

Poison reverse updates
Split horizons prevent routing loops between adjacent routers, but poison reverse updates are necessary to defeat larger routing loops. Generally speaking, increases in routing metrics indicate routing loops. Poison reverse updates then are sent to remove the route and place it in holddown. With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.

IGRP also maintains a number of timers and variables containing time intervals. These include an update timer, an invalid timer, a holddown timer, and a flush timer.

The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds.

The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period.

The holddown timer specifies the amount of time for which information about poorer routes is ignored. The IGRP default for this variable is three times the update timer period plus 10 seconds.

Finally, the flush timer indicates how much time should pass before a route is flushed from the routing table. The IGRP default is seven times the routing update timer.

Today, IGRP is showing its age, it lacks support for variable length subnet masks (VLSM). Rather than develop an IGRP version 2 to correct this problem, Cisco has built upon IGRP's legacy of success with Enhanced IGRP.

Web Links An Introduction to IGRP http://www.cisco.com/en/US/tech/tk365/ tk352/technologies_white_paper09186a0 0800c8ae1.shtml