The speed command
The rate that Ethernet interfaces physically transmit data at is set by the speed command. By default:
| Interface type | Speed |
|---|---|
| Gigabit Ethernet | 1000 Mbps |
| Fast Ethernet | 100 Mbps |
Using speed, we can set a different transmission rate, i.e. speed 10 on a FastEthernet interface will force that interface to transmit at 10 Mbps.
The clock rate command
The clock rate command is used to set the rate at which Serial interfaces physically transmit. By default, serial interfaces transmit at 1.544 Mbps.
If you set clock rate 64000 the Serial interface will transmit at 64 Kbps.
The bandwidth command
This command does not change the physical speed of an interface, although usually it matches the physical speed by default. Thus, for example, setting a bandwidth of 50 Mbps on a Fast Ethernet interface it will still transmit at 100 Mbps.
The use for bandwidth is to modify the software policy of the router, and it affects things like path selection for Link State routing protocols, or how much bandwidth will be guaranteed to a traffic type by QoS.
A common usage for manually setting bandwidth is for serial interfaces, since they default to 1.544 Mbps but the link might require a much lower setting