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!
Chris Bryant, CCIE 12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com), home of free CCNA and CCNP tutorials, The Ultimate CCNA Study Package, Ultimate CCNP Study Packages, and CCNA CBT video training. Pass the CCNA exam , BSCI exam , and BCMSN exam with Chris Bryant, CCIE 12933!
For a copy of his FREE ebooks, How To Pass The CCNA and How To Pass The CCNP , visit the website and download your copies! You can now get a FREE CCNA and CCNP exam question every day with no email registration required. Get your CCNA study guide from The Bryant Advantage!
Bookmark it:
No comments:
Post a Comment