What is Interior Gateway Routing Protocol (IGRP)?

IGRP (Internal Gateway Routing Protocol) is a Dynamic Routing protocol developed by Cisco in the mid-1980s. It is also a routing protocol specific to Cisco devices.

What is Interior Gateway Routing Protocol (IGRP)?

What is IGRP?

Although IGRP and RIP routing protocols are very similar, they have different structures. The RIP has a Maximum of 16 Hop Counts, while the maximum Hop Count of this protocol is 255.

The Distance Vector protocol uses Bandwidth, Delay, Reliability, Load, and MTU (Maximum Transmission Unit) when selecting the best route.

The Administrative Distance (AD) value for this protocol is 100. The AD value of the RIP is 120. It gives priority to RIP. For example, if both Routing protocols are enabled in a network environment, IGRP will take precedence.

If the Bandwith value not set when configuring this routing protocol, the default value of 1.5 Mbit Bandwith evaluate. The biggest difference with RIP is that this protocol uses an AS (Autonomous Number) Number.

The logic of using Bandwith is that it communicates over the fastest connection. In short, on two different connections with 64 Kbit and 1.5 Mbit, the Router sends the packet by selecting the fastest connection (1.5 Mbit).

Only routers with the same AS number can communicate in this protocol. The AS number value is determined by the user according to the network design and layout and is not required.

Protocol Features

1. A Distance Vector protocol.
2. Classful protocol.
3. There is no CIDR support.
4. VLSM has no support.
5. Uses Bandwidth and Delay by default.
6. Uses Reliability and Load.
7. Send a routing table every 90 seconds.
8. The Hop Count value is 100. But it can be set up to 255.
9. Authentication There is no support.
10. Use Triggered Updates.
11. Uses Poison Reverse and Split Horizon techniques.
12. The Administrative Distance value is 100.
13. Used in large networks.

Bandwidth Usage

The following example of bandwidth usage refers to the transmission of packets over the fastest connection.

Configuring Bandwidth Between Cisco Routers

IGRP Timers

1. Update Timer – 90 Seconds.

Sends routing updates every 90 seconds.

2. Invalid Timer – 270 Seconds.

Update Timer x 3 = Invalid Timer

If a network is not reachable, this network is the duration of being invalided from the Router’s routing table.

In short, if the router has not received an update for a route, it will wait before declaring this route invalid.

3. Hold Down Timer – 280 Seconds.

Update Timer x 3 + 10 Seconds= Hold Down Timer

If a network cannot reach or the route metric is increased, it takes to wait before adding the new route. The router does not accept a change to the route until the waiting period expires. Hold Down Timer prevents routing looping (Loop) on the network.

The Hold Down Timer is 280 seconds.

4. Flush Timer – 630 Seconds.

Update Timer x 7 = Flush Timer

The Router uses the Flush Timer to remove the invalid route from the routing table. Flush Timer is 630 seconds by default.

Difference Between IGRP and RIP

If we compare this protocol with RIP, we can better understand the difference between them.

FeatureRIP Version 1RIP Version 2IGRP
Update Timer30 Seconds30 Seconds90 Seconds
MetricHop CountHop CountDefault: Bandwidth, Delay, Reliability, Load, MTU
VLSM SupportNoYesNo
Metric Value16164, 294, 967, 295
ConvergenceSlowSlowSlow

How to Enable?

You can use the following commands in Global mode at the CLI command prompt to enable the IGRP routing protocol on the Cisco Router.

R1# conf t
R1(config)# router igrp 1
R1(config-if)# network 192.168.1.0
R1(config-if)# network 192.168.2.0
R1(config-if)# end
R1#

   Final Word


In this article, we have given brief information about the Dynamic Routing Protocol, Distance Vector IGRP Protocol. It should be noted that Cisco no longer supports this protocol. Cisco recommends the use of the more advanced DV protocol EIGRP. Thanks for following us!

   Related Articles


What is RIP?
What is Dynamic Routing Protocol?
♦ What is Static Route?
♦ What is OSPF?
♦ What is EIGRP?

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