Cisco CCNA Certification Exam Tutorial: Variance And Unequal price Load Balancing

To pass the CCNA exam, you've to understand the way to work with IGRP and EIGRP unequal-cost load balancing. you will not see a lot of IGRP in production networks anymore, however you will see lots of EIGRP, and a part of fine-tuning your EIGRP network is ensuring that each one ways are in use whereas permitting varying bandwidth rates.

Using the variance command is that the straightforward half - it's obtaining the metric that is the onerous half with IGRP. With EIGRP, you simply look within the topology table and that is it. With IGRP, you've to run a debug to induce the proper metric.



The variance command could be a multiplier when the worth provided with the variance command is multiplied by the lowest-cost metric, it should exceed the higher-cost metric so as for the higher-cost route to be added.

If that sounds sophisticated, it's not. It's one in all those things that sounds troublesome, but isn't. Trust me!

In this example, R1 has 2 ways to 172.23.0.0, however is currently using just one. By wanting within the IP routing table, we have 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's another valid path that uses the 172.12.13.0 network, however isn't currently in use.

I 172.23.0.0/16 [100/8576] via 172.12.123.2, 00:00:53, Serial0

IGRP doesn't have a "show" command that displays all valid routes to a destination, as will EIGRP. The command debug ip igrp transactions can show this metric of the routes using the 512 KBPS route.
R1#debug ip igrp transactions
IGRP protocol debugging is on
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
19:17:51: subnet 172.12.13.0, metric 23531 (neighbor 21531)
19:17:51: subnet 172.12.123.0, metric 23531 (neighbor 8476)
19:17:51: network one.0.0.0, metric 24031 (neighbor 8976)
19:17:51: network two.0.0.0, metric 22131 (neighbor 1600)
19:17:51: network three.0.0.0, metric 22031 (neighbor 501)
19:17:51: network 172.23.0.0, metric 21631 (neighbor 1100)
R1(config)#router igrp one
R1(config-router)#variance three
R1#show ip route 172.23.0.0
Routing entry for 172.23.0.0/16
Known via "igrp 1", distance a hundred, metric 8576
Redistributing via igrp one
Advertised by igrp one (self originated)
Last update 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, via Serial1
Route metric is 21631, traffic share count is one
Total delay is 21000 microseconds, minimum bandwidth is 512 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0
172.12.123.3, from 172.12.123.3, 00:00:20 ago, via Serial0
Route metric is 8576, traffic share count is three
Total delay is 21000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 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 three
Total delay is 21000 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 0

The metric for 172.23.0.0 through the direct association is 21631. A variance of three means any route with a metric but the most effective metric multiplied by the variance (in this case, 8576 x three = 25728) are entered into the routing table. R1 currently has 3 unequal-cost ways to 172.23.0.0 in its routing table, and cargo balancing can happen.

IGRP unequal-cost load balancing takes some follow, however as you'll see, once you get the metric it is simple to figure with. simply confirm you recognize the way to get that metric!