Cisco CCNA Certification Exam Tutorial: Variance And Unequal Cost Load Balancing
Posted in Computers on April 30th, 2009 by cahyoTo pass the CCNA exam, you’ve got to know how to work with IGRP and EIGRP unequal-cost load balancing. You may not see much IGRP in production networks anymore, but you’ll see a lot of EIGRP, and part of fine-tuning your EIGRP network is making sure that all paths are in use while allowing for varying bandwidth rates.
Using the variance command is the easy part – it’s getting the metric that’s the hard part with IGRP. With EIGRP, you just look in the topology table and that’s it. With IGRP, you’ve got to run a debug to get the right metric.
The variance command is a multiplier when the value supplied with the variance command is multiplied by the lowest-cost metric, it must exceed the higher-cost metric in order for the higher-cost route to be added.
If that sounds complicated, it’s not. It’s one of those things that sounds difficult, but isn’t. Trust me!
In this example, R1 has two paths to 172.23.0.0, but is currently using only one. By looking in the IP routing table, we’ve seen that the lowest-cost metric for network 172.23.0.0 on R1 is 8576. This path goes through the 172.12.123.0 network. There is another valid path that uses the 172.12.13.0 network, but is not currently in use.
I 172.23.0.0/16 [100/8576] via 172.12.123.2, 00:00:53, Serial0
IGRP does not have a “show” command that displays all valid routes to a destination, as does EIGRP. The command debug ip igrp transactions will show the current metric of the routes using the 512 KBPS route.
19:17:51: IGRP: broadcasting request on Loopback0
19:17:51: IGRP: broadcasting request on Serial0
19:17:51: IGRP: broadcasting request on Serial1
19:17:51: IGRP: received update from 172.12.13.3 on Serial1
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