Configuring IGRP

By arfs -

Routers Used: 2621 w/ CISCO2600-MB-2FE and WIC-2T modules

IOS: c2600-i-mz.121-5.T9

Objective
  • In this lab, configure IGRP for unequal cost load balancing and tune IGRP timers to improve performance.
Scenario
  • International Travel Agency (ITA) has asked for IGRP to be implemented in its WAN.
Step 1
  • Build and configure the network according to the diagram. If the configuration files are used from the previous lab, remove all routing protocols.
    Note: PC1 and PC2 are not required to complete this lab, but they might be used in
    testing or as Telnet clients. If used, the PC1 gateway may be either Dion1 router or
    the Dion2 router.
  • On all three routers, configure IGRP for Autonomous System 24, and enable updates on all active interfaces with the network command.
    Arief(config)#router igrp 24
    Arief(config)#network 192.168.1.0
    Arief(config)#network 192.168.2.0
    Arief(config)#network 192.168.3.0

    Dion1(config)#router igrp 24
    Dion1(config)#network 192.168.1.0
    Dion1(config)#network 10.0.0.0

    Dion2(config)#router igrp 24
    Dion2(config)#network 192.168.2.0
    Dion2(config)#network 10.0.0.0
  • The IGRP metric includes bandwidth in its calculation. Manually configure the bandwidth of serial interfaces in order for metrics to be accurate. Use the following commands to configure the correct bandwidth settings for each serial interface:
    Dion(config)#interface serial 0/0
    Dion1(config-if)#bandwidth 256

    Arief(config)#interface serial 0/0
    Arief(config-if)#bandwidth 256
    Arief(config-if)#interface serial 0/1
    Arief(config-if)#bandwidth 384

    Dion2(config-if)#interface serial 0/0
    Dion2(config-if)#bandwidth 384
  • Use the output from the show interface command to verify the correct bandwidth settings. Use ping and show ip route to verify full connectivity within the network.
Step 2
  • On Arief, configure unequal cost load balancing using the variance 5 command
    Note: A default value of 1 is used for equal cost load balancing. The following are sample
    commands for Arief:
    Arief(config)#router igrp 24
    Arief(config-router)#variance 5
  • Use the show ip route command to verify that the routers are installing two unequal cost routes to the same destination:
    Arief#show ip route
    (output omitted)
    I 10.0.0.0/8 [100/41072] via 192.168.1.2, 00:00:22, Serial0/0
    [100/28051] via 192.168.2.2, 00:00:03, Serial0/1
    C 192.168.1.0/24 is directly connected, Serial0/0
    C 192.168.2.0/24 is directly connected, Serial0/1
    C 192.168.3.0/24 is directly connected, FastEthernet0/0
Step 3
  • On any router, issue the show ip protocols command and check the IGRP invalid, holddown, and flush timers for IGRP.
    Note: A route does not become invalid until after 270 seconds and is not flushed from the
    table until after more than 10 minutes or 630 seconds. Also, the maximum hop count is set
    at 100 by default.
  • In small networks, it is advised that the timers for IGRP be adjusted to speed up the convergence process.
  • Fast IGRP is a specific set of timer settings that result in improved convergence. To configure Fast IGRP, change the IGRP timers as follows:
    • 15 seconds between updates
    • 45 seconds for route expiration
    • 0 seconds for holddown
    • 60 seconds for flushing the route from the table
  • As part of this configuration disable the holddown timers completely. This is done so that after the route for a given network has been removed, a new route for that destination network will be accepted immediately. Finally, reduce the IGRP maximum hop count to a number appropriate to the ITA network.
    Dion1(config)#router igrp 24
    Dion1(config-router)#timers basic 15 45 0 60
    Dion1(config-router)#no metric holddown
    Dion1(config-router)#metric maximum-hops 10
  • Verify the settings with the show ip protocols command.
Step 4
  • In this step, test the IGRP timer settings by simulating a link failure.
  • On Dion1, enable debug so that any changes to the routing table will be reported to the console:
    Dion1#debug ip routing
  • With the connection to Dion1 open, log into Arief. Do this on a separate workstation if
    necessary. On Arief, shut down the Fast Ethernet interface. This will cause the removal of 192.168.3.0 /24 from the Arief routing table.
    Arief(config)#interface fastethernet 0/0
    Arief(config-if)#shutdown
  • Use the show ip route command to verify that Arief no longer possesses a valid route to
    192.168.3.0 /24.
  • Return to Dion1 and issue the show ip route command
    Note: The route to 192.168.3.0 is still in the Dion1 table, but it is flagged as possibly
    down.
    Dion1#sh ip route
    (output omitted)
    C 10.0.0.0/8 is directly connected, FastEthernet0/0
    C 192.168.1.0/24 is directly connected, Serial0/0
    I 192.168.2.0/24 [100/28051] via 10.0.0.2, 00:00:08, FastEthernet0/0
    I 192.168.3.0/24 is possibly down, routing via 10.0.0.2, FastEthernet0/0
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