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Cisco CCNA Certification Test Tutorial: Variation And Unequal Expense Load Balancing

To pass the CCNA exam, you’ve got to know how to deal with IGRP and EIGRP unequal-cost load balancing. You might not see much IGRP in production networks anymore, however you’ll see a lot of EIGRP, and part of fine-tuning your EIGRP network is making certain that all courses are in usage while enabling varying bandwidth rates.

Utilizing the difference command is the easy part – it’s getting the metric that’s the difficult part with IGRP. With EIGRP, you just look in the topology table which’s it. With IGRP, you’ve got to run a debug to get the best metric.

The difference command is a multiplier when the worth supplied with the variance command is increased by the lowest-cost metric, it should go beyond the higher-cost metric in order for the higher-cost path to be included.

It’s not if that sounds complex. It’s one of those things that sounds tough, however isn’t. Believe me!

In this example, R1 has two paths to 172.23.0.0, but is currently using just 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 course goes through the 172.12.123.0 network. There is another legitimate path that uses the 172.12.13.0 network, but is not currently in usage.

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 location, as does EIGRP. The command debug ip igrp transactions will reveal the current metric of the routes using the 512 KBPS route.

R1 #debug ip igrp deals

IGRP procedure debugging is on

19:17:51: IGRP: transmitting demand on Loopback0

19:17:51: IGRP: relaying demand on Serial0

19:17:51: IGRP: transmitting demand on Serial1

19:17:51: IGRP: received upgrade from 172.12.13.3 on Serial1

19:17:51: subnet 172.12.13.0, metric 23531 (next-door neighbor 21531).

19:17:51: subnet 172.12.123.0, metric 23531 (next-door neighbor 8476).

19:17:51: network 1.0.0.0, metric 24031 (neighbor 8976).

19:17:51: network 2.0.0.0, metric 22131 (next-door neighbor 1600).

19:17:51: network 3.0.0.0, metric 22031 (neighbor 501).

19:17:51: network 172.23.0.0, metric 21631 (next-door neighbor 1100).

R1( config) #router igrp 1.
R1( config-router) #variance 3.
R1 #show ip path 172.23.0.0.
Routing entry for 172.23.0.0/ 16.
Understood via “igrp 1”, distance 100, metric 8576.
Redistributing via igrp 1.
Marketed by igrp 1 (self come from).
Last upgrade from 172.12.123.2 on Serial0, 00:00:01 ago.
Routing Descriptor Blocks:.

  • 172.12.13.3, from 172.12.13.3, 00:00:20 ago, by means of Serial1.
    Path metric is 21631, traffic share count is 1.
    Overall hold-up is 21000 split seconds, minimum bandwidth is 512 Kbit.
    Dependability 255/255, minimum MTU 1500 bytes.
    Loading 1/255, Hops 0.
    172.12.123.3, from 172.12.123.3, 00:00:20 ago, through Serial0.
    Route metric is 8576, traffic share count is 3.
    Total hold-up is 21000 microseconds, minimum bandwidth is 1544 Kbit.
    Reliability 255/255, minimum MTU 1500 bytes.
    Filling 1/255, Hops 0.
    172.12.123.2, from 172.12.123.2, 00:00:01 ago, via Serial0.
    Route metric is 8576, traffic share count is 3.
    Overall delay is 21000 microseconds, minimum bandwidth is 1544 Kbit.
    Dependability 255/255, minimum MTU 1500 bytes.
    Filling 1/255, Hops 0.

The metric for 172.23.0.0 through the direct connection is 21631. A difference of 3 means that any route with a metric less than the best metric multiplied by the variance (in this case, 8576 x 3 = 25728) will be participated in the routing table. R1 now has 3 unequal-cost courses to 172.23.0.0 in its routing table, and load balancing will take place.

IGRP unequal-cost load balancing takes some practice, however as you can see, as soon as you get the metric it’s easy to deal with. Simply make certain you know how to get that metric!

Using the variance command is the simple part – it’s getting the metric that’s the difficult part with IGRP. With IGRP, you’ve got to run a debug to get the right metric.

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. The metric for 172.23.0.0 through the direct connection is 21631. A variance of 3 means that any route with a metric less than the best metric multiplied by the variance (in this case, 8576 x 3 = 25728) will be entered into the routing table.

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