All interfaces have default bandwidth values assigned to them. When default bandwidth is much higher the actual bandwidth which can cause misdirect traffic in the network. Maybe there was a better path with 50 Mbps but the routing protocol thought that this path had 100 Mbps available.
The interface bandwidth values not affect the actual speed or capacity of the link. Its is only for routing metric calculation. So, the bandwidth value must reflect the actual speed of the link so that the routing table has accurate best path information.
Usually, the bandwidth values of Ethernet interfaces match the link speed, but some other interfaces and mostly serial interface speed often different than the actual speed of the link. The default bandwidth on most serial interfaces set to 1.544 Mb/s on new Cisco routers and the serial interfaces of older may set to 128 Kbps. The “bandwidth” command tells IOS how much bandwidth is useable on the particular interface. Refer to the example in Figure 1. Where I have made little changes in the previous topology for better understanding. Notice the links between:
- Router1 and Router2 should be set to 10 Mbps
- Router2 and Router3 should be set to 1544 Kbps
- R3 and R4 should be set to 100Mbps
- R1 and R4 should be set to 128 kbps
We can use the “show interfaces” command to view the bandwidth of the interface. The figure2 display the serial interface 0/3/1 settings for Router1. The highlighted area displayed the configured bandwidth. Which is accurate to the link speed. We can check the bandwidth for all interfaces one by one. If bandwidth was over then the actual speed. We can adjust the bandwidth to the actual speed of the link.
Adjusting the Interface Bandwidth
To adjust the bandwidth of the interface use the <bandwidth kilobits> command. If we want to restore the interface default bandwidth then we can use the <no bandwidth> command. Figure-3 illustrates the bandwidth configuration. The serial interface on Router-3 also required the same bandwidth configuration.