Cisco Bandwidth Command vs Clock Rate & Speed Commands

It’s pretty common for individuals, especially new network professionals, to get the two interface commands speed and bandwidth mixed up. Most believe that when applied to an interface, the Cisco bandwidth command and speed command have the same meaning and that the goal is to lower the interface’s throughput to the limit given by bandwidth or speed. The two commands have completely different scopes, and they do not limit the actual speed in any way.

 

Cisco Bandwidth Command

The Cisco bandwidth command is used to communicate the bandwidth value of an interface to higher-level protocols (Ex. routing protocols).

Set the inherited and received bandwidth values for an interface using the bandwidth statement in interface configuration mode. To restore the default bandwidth settings, use the no form of this command.

R1#conf t
R1(config)#int gi0/0
R1(config-if)#bandwidth ?
  <1-10000000>   Bandwidth in kilobits
  inherit        Specify how bandwidth is inherited
  qos-reference  Reference bandwidth for QOS test
  receive        Specify receive-side bandwidth

 

or

R1(config-if)#no bandwidth

 

inherit(Optional) Inherited bandwidth. Specifies how a subinterface inherits the bandwidth of its main interface.
receive(Optional) Receiver bandwidth. Entering this option enables asymmetric transmit/receive operations so that the transmitted (inherit [kbps]) and received bandwidth are different.

 

The Cisco bandwidth command is an optional but frequently used interface command. Despite the name, it is not intended to limit bandwidth, and it cannot be used to change an interface’s actual capacity. The interface bandwidth command is used to inform higher-level protocols about the interface’s speed.

The majority of the time, a routing protocol has to know the interface speed in order to determine the appropriate route. TCP will also change its initial retransmission parameters based on the bandwidth configured on the interface as a result of this operation.

 

Speed Command

The explanation for the speed command is actually simpler than the bandwidth command. You can set the speed on some interfaces (hardware dependent). You can set it to 10 Mbps even if the interface is 100 Mbps. This indicates that the interface is allowed to send packets at speeds of up to 10 Mbps. You may ask, “What if we set the interface speed to 10 Mbps, does that mean OSPF or EIGRP will generate metrics based on that value?” Yes, of course.

What will you do, though, if the speed command has a value other than the normal 10 Mbps or 100 Mbps? 1 Mbps, for example. You can’t change the speed to 1. Or if you have a hardware card that won’t let you go faster than 1 Gbps? That’s why the bandwidth command is used to inform the higher-level protocols about the actual bandwidth capacity.

R1#conf t
R1(config)#int gi0/0
R1(config-if)#speed ?      
  10    Force 10 Mbps operation
  100   Force 100 Mbps operation
  1000  Force 1000 Mbps operation
  auto  Enable AUTO speed configuration

 

or

R1(config-if)# no speed

 

Clock Rate Command

On serial links, this command is used to set the clock rate. While both ends of the serial link can set the clock rate, the router will only accept the command from the DCE end (usually an ISP modem).

R1#conf t
R1(config)#int se0/1/0
R1(config-if)#clock rate 64000

 

or

R1(config-if)#no clock rate

 

NOTE
The clock rate specifies how many bits can be transmitted at a certain period. When the clock rate is set to 64000, you’ll never be able to send more than 64 Kbps through the interface.

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