CCNA Certification Training: IGRP And Equal Cost Load Sharing
By Chris Bryant, CCIE #12933
The Cisco-proprietary protocols IGRP and EIGRP assume all Serial interfaces are connected to a T1 line, which runs at 1544 kbps. If a serial line is actually configured to a slower line, IGRP has no way to detect this. The bandwidth value should be changed at the interface level to reflect the correct speed, allowing IGRP to recalculate the routes and give the routers in the IGRP AS a truer picture of the network.
We'll use the following network to illustrate this concept:
Assume the direct link between R1 and R3 is only a 512 KBPS line, and the Frame Relay connection is a true T1 line. By default, IGRP calculates its metrics by considering both of these connections to be a T1 line running at 1544 kbps, which results in equal-cost load balancing for traffic leaving R1 destined for 172.23.23.0.
Load balancing is occurring on the equal-cost routes, but since one of the lines is not truly running at 1544 KBPS, this load balancing may not be desired.
The interface-level command bandwidth 512 on both ends of the R1 – R3 direct connection allows IGRP to recalculate the metric based on the actual bandwidth of the interfaces, rather than the assumed value. After configuring the true bandwidth, show ip route 172.23.0.0 shows load balancing is only occurring over the true T1 lines.
R1(config)#interface serial1
R1(config-if)#bandwidth 512
R3(config)#interface serial1
R3(config-if)#bandwidth 512
This is a command you really have to be careful with, because your first instinct may be to enter bandwidth 512000. IOS Help reminds us that the value for this particular command must be entered in KBPS.
R1(config)#int s1
R1(config-if)#bandwidth ?
<1-10000000> Bandwidth in kilobits
You're going to see some Cisco commands that want a value in BPS, others in MBPS. The key is to always use IOS Help to check -- it takes five seconds to do so, and saves you a lot of troubleshooting time!
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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