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To 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.
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 1.0.0.0, metric 24031 (neighbor 8976)
19:17:51: network 2.0.0.0, metric 22131 (neighbor 1600)
19:17:51: network 3.0.0.0, metric 22031 (neighbor 501)
19:17:51: network 172.23.0.0, metric 21631 (neighbor 1100)
R1(config)#router igrp 1
R1(config-router)#variance 3
R1#show ip route 172.23.0.0
Routing entry for 172.23.0.0/16
Known via "igrp 1", distance 100, metric 8576
Redistributing via igrp 1
Advertised by igrp 1 (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 1
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 3
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 3
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 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. R1 now has three unequal-cost paths
to 172.23.0.0 in its routing table, and load balancing will take place.
IGRP unequal-cost load balancing takes some practice, but as you can
see, once you get the metric it's easy to work with. Just make sure you
know how to get that metric! |