IGRP Illustration

johnny_jiang

                                It is useful to understand how IGRP works, it is obsolete though.
First of all, IGRP has a lot in common with RIP. They are both classful distance-vector routing protocols as no netmask field in update packet. They do both advertise route with AD=0 no matter if static or connected.
IGRP provides a number of features that are
designed to enhance its stability. These include holddowns, split
horizons, and poison-reverse updates.
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.
They also both follow a rule when sending and receiving updates. See the detail from the following link:
http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a0080093fd8.shtml
IGRP packet format
==================

• Interior Routes - if there are subnets then these are advertised.  These routes are subnets that
  are locally connected.
• System Routes - routes to summarised networks within an AS.
• Exterior Routes - routes to networks outside the AS and are called default networks.

Usually, we use system routes and exterior routes. Exterior route is used to advertise update for candidate default. In the snapshot above, 192.168.3.0 will be marked with asterisk as candidate default.
Destination (3 octets) - Destination network, just containing the last
three octets for interior routes (e.g. 24.5.0 for the network
10.24.5.0) since the first octet will be known. For System and External
routes, the routes would have been summarised so the last octet will
always be zero (e.g. 10.24.5.0 will be entered as 10.24.5).
Please understand that IGRP does not trigger any flash update when route changed by commands such as 'network', 'redistribute' etc, that is, the updated routes will be advertised the interval timer times out. (Next time sending update) Certainly, the physical interface failure would cause triggered updates.
Split horizon
=============
IGRP is like RIP, it will follow split horizon when redistributing static route.
Default route
=============
Because it's special packet form, IGRP does not support 0.0.0.0 route. We have several ways to generate candidate default.
1. Use commands 'ip default-network' and 'network' to advertise one connected/static (AD=0) default route.
Please note it must be declared by 'network' command, not like RIP.
2. Use 'ip default-network' to mark igrp-advertised route or redistributed route, then that route will be changed from system route to exterior route.
06:52:36: IGRP: sending update to 255.255.255.255 via Serial1/1 (192.168.20.2)
06:52:36:       network 220.1.1.0, metric=26476
06:52:36:       network 192.168.1.0, metric=8476
06:52:36:       network 192.168.2.0, metric=10476
06:52:36:       exterior 10.0.0.0, metric=10476
               
               
               
               
               
               
               

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