Introduction to Bandwidth Metric

EIGRP and OSPF both are using bandwidth metric which is a static value for calculating routing metrics. The bandwidth is displayed in kilobits per second (kb/s). The default bandwidths for different interfaces are:

• Serial Interface – 1544 kb/s
• Ethernet is – 10,000 kb/s
• FastEthernet – 100,000
• GigaEthernet – 1,000,000
• TenGig – 10,000,000

Figure 1 illustrates the topology we have used throughout this section. We can verify the bandwidth with the “show interfaces” command. The default value of the bandwidth may or may not reflect the real physical bandwidth of the interface. If real bandwidth of the link is different from the default bandwidth value, the bandwidth value should be modified.

Configuring the Bandwidth Parameter

Most of the links uses the bandwidth metric defaults values as written above. The correct value for bandwidth is very important to the accuracy of routing information. To modify the bandwidth the command syntax is as under:

Router(config-if)# bandwidth <kilobits-bandwidth-value>

If the bandwidth is already changed from the default value and you want to restore the default bandwidth you can use the “no bandwidth” command to restore the default value. In topology, the link between R1 and R2 has a bandwidth of 1544 kb/s, and the link between R1 and R3 has a bandwidth of 10,000 kb/s.

Verifying the Bandwidth Parameter

We can verify the interface bandwidth using the show interfaces command”, as shown in Figure 2. Modifying the bandwidth value on the interface does not change the real bandwidth of the link. The bandwidth only modifies the bandwidth metric used by routing protocols, such as EIGRP and OSPF.