Interior Gateway Routing Protocol (IGRP)

Interior Gateway Routing Protocol (IGRP) is a standards-based, distance-vector, interior gateway protocol (IGP) used by routers to exchange routing information.  IGRP uses a composite metric of bandwidth and delay to determine the best path between two locations.  The metric can also be administratively configured to factor in the Maximum Transmission Unit (MTU), Reliability, and load for the link. In a IGRP network, each router broadcasts its entire IGRP table to its neighboring routers every 90 seconds.  When a router receives a neighbor's IGRP table, it uses the information provided to update its own routing table and then sends the updated table to its neighbors. This procedure is repeated by each router and results in a state referred to as network convergence, in which all routers have an identical view of the internetwork topology.

P Addresses: Please set these IP addresses on the interfaces of your routers.

Lets connect to Router1 and get it configured.  We will be using the table above for our IP addresses.

Router>en
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#int e0
Router(config-if)#ip address 10.1.1.1 255.255.255.0
Router(config-if)#no shut
Router(config-if)#exit
00:35:15: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up
Router(config)#hostname Router1
Router1(config)#int s0
Router1(config-if)#ip address 172.16.10.1 255.255.0.0
Router1(config-if)#no shut
00:35:16: %LINK-3-UPDOWN: Interface Serial0, changed state to up
Router1(config-if)#exit
00:35:16: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed sta
te to up

IGRP is classful, meaning it does not include the subnet mask in its routing table updates. So now lets go ahead and start the lab.

1. We first want to configure Router1 for IGRP. To enable IGRP as the routing protocol we only need to type: router IGRP AS.  The AS stands for a Autonomous System number.  An Autonomous System is defined as a network under a common administration with a common routing policy. You will need to use the SAME autonomous system number on every router that you would like to share its routing table with.  We can see this below in the router output. Notice the new mode we have entered Router1(config-router)# that tells us we are configuring the router.  

Router1(config)#router IGRP 100
Router1(config-router)#

Now that we have IGRP running on our Router we need to tell the router which networks it is connected to. We do this by using the network statement.  What this means is every interface of our router that is directly connected to an active network needs a network number. We will have some networks using the same ip addressing schemes with different subnets, and some are using entirely different addressing schemes.  Look at the diagram below. In this diagram we have three different kinds of addressing schemes.  Lets look at these in more detail.  On Router 1 we have an IP address of 10.1.1.1 with a /24 subnet mask.  Since IGRP is classful you are only required to enter the class part of the address for the network statement.  For example on Router1 we have already issued the command router IGRP, we then need to specify the directly connected networks to Router1 so the router can advertise these routes in its routing table.  To do this we would only need to type: network 10.0.0.0   now we have not told the router about the network on his serial interface, to do this we would type: network 172.16.0.0  Lets look at Router 2 what network statement would we need to use on this router ______________________________________(see the answer below the diagram.)

The answer is network 10.0.0.0. The network statement for the ethernet link is the same for Router1 and Router2.  On router1 what network statement would you need for the serial link? For this network statement you used the classful portion of the address 172.16.10.1 which would be just network 172.16.0.0.

Now that we understand the network command lets enter it on our Router1.

Router1(config-router)#network 172.16.0.0
Router1(config-router)#network 10.0.0.0
Router1(config-router)#

If you notice we only needed to enter 10.0.0.0 for our network statement, this is because 10.0.0.0 is a Class B address and IGRP only uses the classful portion of the address.  Now we have configured Router1 for IGRP lets connect to Router2 and get it setup.

 We need to connect to Router2 and follow the same instructions.  Lets select Router2 from the Window pull down menu.  When we connect we are going to set a hostname to Router2, then set the ip addresses to the table above and configure IGRP.  

Router>en
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#hostname Router2
Router2(config)#int e0
Router2(config-if)#ip address 10.1.1.2 255.255.255.0
Router2(config-if)#no shut
Router2(config-if)#exit
01:23:17: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up
01:23:18: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up
Router2(config)#

Now add the IGRP stuff!

Router2(config)#router IGRP 100
Router2(config-router)#network 10.0.0.0
Router2(config-router)#exit
Router2(config)#exit
Router2#

We should now have IGRP running on our network between Router1 and Router2.  We need to get Router4 setup.

We need to connect to Router4 and follow the same instructions.  Lets select Router4 from the Window pull down menu.  When we connect we are going to set a hostname to Router4, then set the ip addresses to the table above and configure IGRP.  

Router>en
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#hostname Router2
Router4(config)#int s0
Router4(config-if)#ip address 172.16.10.2 255.255.0.0
Router4(config-if)#no shut
Router4(config-if)#exit
01:23:17: %LINK-3-UPDOWN: Interface Ethernet0, changed state to up
01:23:18: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0, changed state to up
Router4(config)#

Now add the IGRP stuff!

Router4(config)#router IGRP 100
Router4(config-router)#network 172.16.0.0
Router4(config-router)#exit
Router4(config)#exit
Router4#

  Now that we have IGRP running on our entire network lets verify that it is receiving routes.  To do this we will be using some show commands.  The most common one is show ip route.  This displays all entries in the routing table.  If we do this on our Router B we will see the route to our directly connected Router1.  Lets take a look at our routing table, to do this type: show ip route from the privilege mode.

Lets look at the first entry I  10.1.1.0/24 [100/1] via 172.16.10.2, 00:00:21, Serial0.  It starts off with I this says it is a IGRP route it then says the destination network with sunbet mask in this case it is 10.1.1.0 with a /24 (255.255.255.0) subnet mask.  Next it gives 100/1 the 100 is the administrative distance, IGRP's default administrative distance is 100.  Administrative distance is considered the trustworthiness of the route. If you have two routing protocols with the same route the router will pick the route with the lower number.  The 1 is the hops required to get to the destination network.  The next piece of information is the via 172.16.10.1 that is the next hop address it must go to.  The last item is that this information was learned via Serial0.  

Another great command is show ip protocols.  This displays information about the ip routing protocols you have enabled.  Lets type the command : show ip protocols and see what we get.

Router4#show ip protocols
Routing Protocol is igrp 100
Sending updates every 90 seconds, next due in 12 seconds
Invalid after 270 seconds, hold down 280, flushed after 630
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
IGRP maximum hopcount 100
IGRP maximum metric variance 1
Redistributing: igrp 100
Routing for Networks:
172.16.0.0
Routing Information Sources:
Gateway Distance Last Update
172.16.10.2 100 00:00:09
Distance: (default is 100)

Router4#

Looking at the output in detail we see we are sending updates every 90 seconds.  We know IGRP is a distance vector routing protocol so it exchanges its entire routing table every 90 seconds.  We also see our network statements are working by noticing the networks are both under the Routing for Networks area.  The last area to notice is the Distance which we said was administrative distance.  This tells us the default is 100 and that is what we are using. 

Conclusion:

In this lab we have configured our routers for IGRP so that we can exchange information with more than the directly connected neighbor.  We have learned that IGRP's metric is hop count and the routers send updates every 30 seconds be default.  Now in the next lab we will go into IGRP (Interior Gateway Routing Protocol).

Copyright (c) 2001 Boson Software, Inc.  All Rights Reserved.