Cisco CCNA Certification Exam Tutorial Variance And Unequal Cost Load Balancing
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Cisco CCNA Certification Exam Guide: Variance and Unequal Cost Load Balancing
Introduction
Navigating Cisco's CCNA exam requires a solid understanding of IGRP and EIGRP, especially unequal-cost load balancing. While IGRP may be rare in today's production networks, EIGRP is prevalent, and mastering it involves ensuring all paths are utilized efficiently, even with differing bandwidths.Using the Variance Command
The concept of the variance command is straightforward, yet determining the right metric for IGRP can be complex. In the case of EIGRP, simply check the topology table. However, with IGRP, you need to run a debug to find the correct metric.The variance command acts as a multiplier. By multiplying the lowest-cost metric by this value, the result must exceed the higher-cost metric for the higher-cost route to be included.
If this seems complicated, don't worry. It’s easier than it sounds. Trust me!
Example Scenario
Consider this scenario: Router R1 has two paths to the network 172.23.0.0 but currently utilizes only one. The IP routing table shows the lowest-cost metric for this network on R1 as 8576, routing through the 172.12.123.0 network. A valid alternate path exists via the 172.12.13.0 network but isn't in use.Example:
```
I 172.23.0.0/16 [100/8576] via 172.12.123.2, 00:00:53, Serial0
```
Determining Metrics with IGRP
Unlike EIGRP, IGRP lacks a command to view all valid routes to a destination. Instead, use `debug ip igrp transactions` to reveal current metrics using a 512 Kbps route.Commands:
```
R1
debug ip igrp transactions
```Sample output:
```
IGRP protocol debugging is on
19:17:51: Received update from 172.12.13.3 on Serial1
Subnet 172.12.13.0, metric 23531 (neighbor 21531)
```
Configuring Variance
To enable unequal-cost load balancing, configure variance on R1, ensuring multiple paths are used:Commands:
```
R1(config)
router igrp 1
R1(config-router)variance 3
R1show ip route 172.23.0.0
```With a variance of 3, any route with a metric less than the best metric multiplied by the variance (8576 x 3 = 25728) is added to the routing table. This allows R1 to have three unequal-cost paths to 172.23.0.0, enabling effective load balancing.
Conclusion
Mastering IGRP unequal-cost load balancing might require some practice, but once you grasp the metric, it becomes straightforward. Ensure you know how to obtain that crucial metric to succeed in your networking endeavors.You can find the original non-AI version of this article here: Cisco CCNA Certification Exam Tutorial Variance And Unequal Cost Load Balancing.
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