Features of IGRP

A thorough understanding of the IGRP protocol is required to successfully isolate and troubleshoot problems pertaining to IGRP networks. The different features of IGRP are shown in Table 7.1.

Table 7.1 : Features of the IGRP Routing Protocol

Features	IGRP Values
Protocol type	Distance vector
Subnet information	Classful
Metric	Composite
Count to infinity	100 by default
Routing updates mode	Broadcast
Flash or triggered updates	Yes
Load balancing	Up to four paths, by default
Algorithm	Bellman-Ford

IGRP requires timers for different routing operations. These IGRP timers are set to default values that can be changed, if required. Table 7.2 lists the default timer values used by IGRP.

Table 7.2: IGRP Timers

Timer	Function	Default Value
Update	Specifies how frequently routing update messages should be sent.	90 seconds.
Invalid	Specifies how long a router should wait in the absence of routing updates about a route before declaring it invalid.	270 seconds. Three times update timer.
Hold-down	Specifies duration given to routes learned and registered in the routing table of the router.	280 seconds. 10 seconds more than invalid timer.
Flush	Specifies how much time should pass before a route should be flushed from the routing table.	630 seconds. Seven times update timer.

The IGRP metrics used to calculate the best path to reach a destination are:

Bandwidth: Difference between highest and lowest frequencies available for network signals. Bandwidth value ranges from 1.2 kbps to 10 gbps.

Delay: Time taken for packets to move from source to destination. Delay is expressed in units of 10 microseconds.

Load: Amount of router resource used. Load is measured in a scale of 1 to 255.

Reliability: Ratio of expected keepalives to received keepalives from a link. Reliability is expressed in a scale of 1 to 225.

MTU: Maximum packet size that an interface can handle. MTU is expressed in bytes.

Interior route: Directly connected to a router interface.

System route: Learned by other IGRP enabled routers configured with the same Autonomous System Number (ASN).

Exterior route: Learned or redistributed by other IGRP enabled routers configured with a different ASN.