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 172.23.0.016 1008576 Via 172.12.123.2, 00:00:53, Serial0IGRP 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. R1debug Ip Igrp TransactionsIGRP Protocol Debugging Is On19:17:51: IGRP: Broadcasting Request On Loopback019:17:51: IGRP: Broadcasting Request On Serial019:17:51: IGRP: Broadcasting Request On Serial119:17:51: IGRP: Received Update From 172.12.13.3 On Serial119:17:51: 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 1R1(config-router)variance 3R1show Ip Route 172.23.0.0Routing Entry For 172.23.0.016Known Via "igrp 1", Distance 100, Metric 8576Redistributing Via Igrp 1Advertised By Igrp 1 (self Originated)Last Update From 172.12.123.2 On Serial0, 00:00:01 AgoRouting Descriptor Blocks: 172.12.13.3, From 172.12.13.3, 00:00:20 Ago, Via Serial1Route Metric Is 21631, Traffic Share Count Is 1Total Delay Is 21000 Microseconds, Minimum Bandwidth Is 512 KbitReliability 255255, Minimum MTU 1500 BytesLoading 1255, Hops 0172.12.123.3, From 172.12.123.3, 00:00:20 Ago, Via Serial0Route Metric Is 8576, Traffic Share Count Is 3Total Delay Is 21000 Microseconds, Minimum Bandwidth Is 1544 KbitReliability 255255, Minimum MTU 1500 BytesLoading 1255, Hops 0172.12.123.2, From 172.12.123.2, 00:00:01 Ago, Via Serial0Route Metric Is 8576, Traffic Share Count Is 3Total Delay Is 21000 Microseconds, Minimum Bandwidth Is 1544 KbitReliability 255255, Minimum MTU 1500 BytesLoading 1255, Hops 0The 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!
