Information Technology Reference
In-Depth Information
not always the most appropriate path. For example, between two paths to a destination
network—one with two 56-kbps links and another with four T1 links—the router chooses
the first path because of the lower number of hops (see Figure10-3). However, this is not
necessarily the best path. You would prefer to transfer a 20-MB file via the T1 links rather
than the 56-kbps links.
Path 1: 3 Hop Counts
56 k
56 k
T-1
T-1
T-1
T-1
Path 2: 5 Hop Counts
Figure 10-3
Hop Count Metric
Bandwidth
The bandwidth parameter uses the interface bandwidth to determine a best path to a des-
tination network. When bandwidth is the metric, the router prefers the path with the high-
est bandwidth to a destination. For example, a Fast Ethernet (100 Mbps) is preferred over
a DS-3 (45 Mbps). As shown in Figure 10-3, a router using bandwidth to determine a path
would select Path 2 because of the larger bandwidth, 1.5 Mbps over 56 kbps.
If a routing protocol uses only bandwidth as the metric and the path has several different
speeds, the protocol can use the lowest speed in the path to determine the bandwidth for
the path. EIGRP and IGRP use the minimum path bandwidth, inverted and scaled, as one
part of the metric calculation. In Figure 10-4, Path 1 has two segments, with 256 kbps
and 512 kbps of bandwidth. Because the smaller speed is 256 kbps, this speed is used as
Path 1's bandwidth. The smallest bandwidth in Path 2 is 384 kbps. When the router has
to choose between Path 1 and Path 2, it selects Path 2 because 384 kbps is larger than
256 kbps.
Path 1: BW - 256 kbps
256 k
512 k
512 kbps
384 kbps
T-1
768 kbps
Path 2: BW - 384 kbps
Figure 10-4
Bandwidth Metric Example
