Information Technology Reference
In-Depth Information
Cost
Cost is the name of the metric used by OSPF and IS-IS. In OSPF on a Cisco router, a link's
default cost is derived from the interface's bandwidth.
Cisco's implementation of IS-IS assigns a default cost of 10 to all interfaces.
The formula to calculate cost in OSPF is
10
8
/BW
where BW is the interface's default or configured bandwidth.
For 10-Mbps Ethernet, cost is c alc ulated as follows:
BW = 10 Mbps = 10 * 10
6
= 10,000,000 = 10
7
cost (Ethernet) = 10
8
/ 10
7
= 10
The sum of all the costs to reach a destination is the metric for that route. The lowest cost
is the preferred path.
Figure 10-5 shows an example of how the path costs are calculated. The path cost is the
sum of all costs in the path. The cost for Path 1 is 350 + 180 = 530. The cost for Path 2 is
15 + 50 + 100 + 50 = 215.
Path 1: Cost = 350 + 180 = 530
350
180
15
50
50
100
Path 2: Cost = 15 + 50 + 100 + 50 = 215
Figure 10-5
Cost Metric Example
Because the cost of Path 2 is less than that of Path 1, Path 2 is selected as the best route to
the destination.
Load
The load parameter refers to the degree to which the interface link is busy. The router
keeps track of interface utilization; routing protocols can use this metric when calculating
the best route. Load is one of the five parameters included in the definition of the EIGRP
metric. By default, it is not used to calculate the composite metric. If you have 512-kbps
and 256-kbps links to reach a destination, but the 512-kbps circuit is 99 percent busy and
the 256-kbps is only 5 percent busy, the 256 kbps link is the preferred path. On Cisco
routers, the percentage of load
