Routing protocols use metrics to determine best routes to a destination. Some routing protocols use a combination of metrics to build a composite metric for best path selection. This section describes metrics and covers routing loop-prevention techniques. You must understand each metric for the ENSLD 300-420 exam.
The following are some common routing metric parameters:
- Hop count
- Bandwidth
- Cost
- Load
- Delay
- Reliability
- Maximum transmission unit (MTU)
Hop Count
The hop count parameter counts the number of links between routers that a packet must traverse to reach a destination. The RIP routing protocol uses hop count as the metric for route selection. If all links were the same bandwidth, this metric would work well. The problem with routing protocols that use only this metric is that the shortest hop count is 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 Figure 3-3). 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.
Figure 3-3 Hop Count Metric
Bandwidth
The bandwidth parameter uses the interface bandwidth to determine the best path to a destination network. When bandwidth is the metric, the router prefers the path with the highest bandwidth to a destination. For example, a Fast Ethernet (100 Mbps) link is preferred over a DS-3 (45 Mbps) link. As shown in Figure 3-3, a router using bandwidth to determine a path would select Path 2 because of the greater 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 3-4, 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.
Figure 3-4 Bandwidth Metric Example