The Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary protocol. Like RIP, IGRP is a distance-vector interior routing protocol. However, unlike RIP, IGRP can be used in larger autonomous systems due to its large maximum hop-count limit of 255, compared to RIP’s maximum hop count of 16.
IGRP uses bandwidth and delay of the line by default as metric for determining the best route to an internetwork. This is called a composite metric. Reliability, load and maximum transmission unit (MTU) can also be used, although they are not used by default.
To control performance IGRP uses different kind of timers:
Update Timers specifies how frequently IGRP routing messages will be sent. The default is 90 seconds.
Invalid Timer specifies how long a router should wait in the absence of a routing-update message of a specific route before declaring it invalid. The default is three times the Update timer, 270 seconds.
Holddown Timer specifies the holddown period. The default is three times the update timer plus 10 seconds, 280 seconds.
Flush Timer indicates how much time should pass before an IGRP route is flushed from the routing table. The default is seven times the routing update period, 630 seconds.
IGRP Configurations
Configuring IGRP is similar to configuring RIP in that after the router command you must specify only directly connected (system routes) networks. The only difference is in the command to enable the routing protocol. You must specify an AS number when enabling IGRP. The AS number parameter specifies the autonomous system number that is supported by this IGRP process and allows multiple IGRP processes to run on a single router. The AS number can be between 1 and 65,655.
For example:
RTR(config)# router igrp 10
RTR(config-router)# network 200.40.0.0
RTR(config-router)# network 200.30.0.0
Monitoring and Verifying IGRP
| Command | Description |
| Show ip protocols | Shows routing protocol parameters and current timer values |
| Debug ip igrp transactions | Issues log messages with details of the IGRP updates. |
| Debug ip igrp events | Issues log messages for each igrp updates |
| Ping | Sends and receive ICMP echo messages to verify connectivity |
| trace | Sends a series of ICMP echoes with increasing TTL value |
| Show ip route | Shows routing protocol parameters and current timer values |
IGRP Configuration
RouterA#show ip route
Gateway of last resort is not set
C 1.0.0.0/8 is directly connected, serial0/1
C 3.0.0.0/8 is directly connected, serial0/0
C 192.168.1.0/24 is directly connected, loopback0
Configure igrp on Router A
RouterA#configure terminal
Enter configuration commands, one per line. End with Cntl/z
RouterA#(config)#router igrp 10
RouterA#(config-router)#network 1.0.0.0
RouterA#(config-router)#network 3.0.0.0
RouterA#(config-router)#network 192.168.1.0
RouterA#(config-router)#exit
RouterA#(config)#
Now verify these configuration on router A with command us under:
RouterA#show ip route
To check the routing protocol with this command.
RouterA#show ip protocols
RouterB#show ip route
Gateway of last resort is not set
C 1.0.0.0/8 is directly connected, serial0/1
C 2.0.0.0/8 is directly connected, serial0/0
C 192.168.2.0/24 is directly connected, loopback0
Configure igrp on Router B
RouterB#configure terminal
RouterB#(config)#router igrp 10
RouterB#(config-router)#network 1.0.0.0
RouterB#(config-router)#network 2.0.0.0
RouterB#(config-router)#network 192.168.2.0
RouterB#(config-router)#exit
RouterB#(config)#
Now verify these configuration on router B with command us under:
RouterB#show ip route
RouterC#show ip route
Gateway of last resort is not set
C 2.0.0.0/8 is directly connected, serial1
C 3.0.0.0/8 is directly connected, serial0
C 192.168.3.0/24 is directly connected, loopback0
Configure igrp on Router C
RouterC#configure terminal
RouterC#(config)#router igrp 10
RouterC#(config-router)#network 2.0.0.0
RouterC#(config-router)#network 3.0.0.0
RouterC#(config-router)#network 192.168.3.0
RouterC#(config-router)#exit
RouterC#(config)#
Now verify these configuration on router C with command us under:
RouterC#show ip route
Tags:cisco, cisco router, IGRP, IGRP Configurations, Interior Gateway Routing Protocol, routing protocol
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