EIGRP Defaults in Bandwidth Utilization

EIGRP will not use more than 50 percent of the stated bandwidth on a link. The bandwidth command used on the interfaces of a Cisco router allows the default settings on links to be overridden. This is often necessary on serial links because the default bandwidth is 1.544 Mbps or a T1. If in reality the link is 56 kbps, it is easy to see how EIGRP could saturate the link. EIGRP will try to use 50 percent of a T1 link (772 kbps), far exceeding the real bandwidth of the line. This will mean not only the dropping of data packets due to congestion, but also the dropping of EIGRP packets. This will cause confusion in the network, not to mention miscalculated routes, retransmission, and user irritation as the network slows.

It is essential to configure all interfaces to reflect the true speed of the line.

NOTE Other technologies on a Cisco router will use this value to make decisions. Therefore, ensure that the bandwidth stated is indeed the speed of the link. When you issue the show interface command, the configured bandwidth of the link will be shown along with a field identifying the load on the line. The load is the amount of traffic sent out of the interface, proportional to the bandwidth of the link, in which the bandwidth is the stated bandwidth and not the speed of the interface.

If it is necessary to artificially lower the bandwidth command, this should be done in consideration of the other network applications.

The bandwidth is a logical construct whose value can have wide-reaching implications on the functioning of your network. It does not affect the actual speed of the link.

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