Stability Features of IGRP Protocol

IGRP provides a number of features that are designed to enhance its stability.These include hold downs,split horizons,and poison-reverse updates.Hold downs are used to prevent regular update messages from inappropriately reinstating a route that might have gone bad.When a router goes down, neighboring routers detect this via the lack of regularly scheduled update messages.These routers then calculate new routes and send routing update messages to inform their neighbors of the router change. This activity begins a wave of triggered updates that filter through the network.These triggered updates do not instantly arrive at every network device.Thus,it is possible for a device that has yet to be informed of a network failure to send a regular update message,which advertises a failed route as being valid to a device that has just been notified of the network failure.In this case, the latter device would contain (and  potentially advertise)incorrect routing information. Hold downs tell routers to hold down any changes that might affect routes for some period of time. The holddown period usually is calculated to be just greater than the period of time necessary to update the entire network with a routing change.

Split horizons derive from the premise that it is never useful to send information about a route back in the direction from which it came.illustrates the split-horizon rule.Router 1(R1)advertises that it has a route to Network A.There is no reason for Router 2 (R2) to include this route in its update back to R1 because R1 is closer to Network A.The split-horizon rule says that R2 should strike this route from any updates that it sends to R1.The split-horizon rule helps prevent routing loops.Consider,for example, the case in which R1’s interface to Network A goes down. Without split horizons, R2 continues to inform R1 that it can get to Network A (through R1).If R1 does not have sufficient intelligence,it actually might pick up R2’s route as an alternative to its failed direct connection,causing a routing loop.Although holddowns should prevent this,split horizons are implemented in IGRP because they provide extra algorithm stability.Split horizons should prevent routing loops between adjacent routers,but poison-reverse updates are necessary to defeat larger routing loops.Increases in routing metrics generally indicate routing loops.

Poison-reverse updates then are sent to remove the route and place it in holddown. In Cisco’s implementation of IGRP, poison-reverse updates are sent if a route metric has increased by a factor of 1.1 or greater

Timers

IGRP maintains a number of timers and variables containing time intervals.These include an update
timer,an invalid timer,a hold-time period,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 hold-time variable specifies the hold down period.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 should be flushed from the routing table.The IGRP default is seven times the routing update period