Route Metrics and Best Route Selection

In the following sections, you will learn about the OSPF metric and how it is calculated.

Calculating the OSPF Metric

The OSPF metric is commonly referred to as the cost. The cost is derived from the bandwidth of a link using the formula 10⁸ / bandwidth (in bps). This means that different links are assigned different cost values, depending on their bandwidth. Using this formula, the OSPF cost of a 10Mbps Ethernet interface would be calculated as follows:

• Cost = 10⁸ / bandwidth (bps)
• Cost = 100 000 000 / 10 000 000
• Cost = 10

Using the same formula, the OSPF cost of a T1 link would be calculated as follows:

• Cost = 10⁸ / bandwidth (bps)
• Cost = 100 000 000 / 1 544 000
• Cost = 64.77

NOTE: When calculating the OSPF metric, point math is not used. Therefore, any such values are always rounded down to the nearest integer. Regarding the previous example, the actual cost for a T1 link would be rounded down to 64.

The OSPF cost of an interface can be viewed using the show ip ospf interface [name] command, as was illustrated previously. The default reference bandwidth used in metric calculation can be viewed in the output of the show ip protocols command, as is illustrated in the following output:

The default reference bandwidth used in OSPF cost calculation can be adjusted using the auto-cost reference-bandwidth <1-4294967> router configuration command and specifying the reference bandwidth value in Mbps. This is particularly important in networks that have links that have a bandwidth value over 100Mbps, for example, GigabitEthernet links. In such networks, the default value assigned to the GigabitEthernet link would be the same as that of a FastEthernet link. In most cases, this is certainly not desirable, especially if OSPF attempts to load balance across both links.

To prevent this skewed calculation of cost value, the auto-cost reference-bandwidth 1000 router configuration command should be issued on the router. This results in a recalculation of cost values on the router using the new reference bandwidth value. For example, following this configuration, the cost of a T1 link would be recalculated as follows:

• Cost = 10⁹ / bandwidth (bps)
• Cost = 1 000 000 000 / 1 544 000
• Cost = 647.66

NOTE: Again, because the OSPF metric does not support point values, this would be rounded down to a metric value of simply 647, as illustrated in the following output:

When the auto-cost reference-bandwidth 1000 router configuration command is issued, Cisco IOS software prints the following message indicating that this same value should be applied to all routers within the OSPF domain. This is illustrated in the following output:

While this may seem like an important warning, keep in mind that the use of this command simply affects the local router. It is not mandatory to configure it on all routers; however, for exam purposes, ensure that a consistent configuration is implemented on all routers.

Influencing OSPF Metric Calculation

The calculation of the OSPF metric can be directly influenced by performing the following:

• Adjusting the interface bandwidth using the bandwidth command
• Manually specifying a cost using the ip ospf cost command

The use of the bandwidth command was described in a previous module when we discussed EIGRP metric calculation. As stated earlier, the default OSPF cost is calculated by dividing the link bandwidth by a reference bandwidth of 10⁸, or 100 Mbps. Either incrementing or decrementing the link bandwidth directly affects the OSPF cost for the particular link. This is typically a path control mechanism used to ensure that one path is preferred over another.

However, as was described in the previous module, the bandwidth command affects more than just the routing protocol. It is for this reason that the second method, manually specifying a cost value, is the recommended method for influencing OSPF metric calculation.

The ip ospf cost <1-65535> interface configuration command is used to manually specify the cost of a link. The lower the value, the greater the probability that the link will be preferred over other links to the same destination network but with higher cost values. The following example illustrates how to configure an OSPF cost of 5 for a Serial (T1) link:

This configuration can be validated using the show ip ospf interface [name] command, as illustrated in the following output: