Networks Horizon

Bandwidth and Clockrate-Difference


Clockrate is actual line speed and is configured on serial links in DCE side of network. When you set the clock rate for a serial interface, you are setting the speed of the interface,in other words, actual rate of data transfer.


Bandwidth is the value that you allocate to the link at DTE site to help with routing decision and QOS. The bandwidth command however does not adjust the speed of the line at all but it should be configured on ALL DTE and DCE interfaces because it is used by;

1. Routing protocols to identify the routing cost and identify best path.
2. QOS (Quality of Service) to identify how much bandwidth is availble to prioritize/shaping/policing.

Configuring wrong bandwidth will impact routing decisions and QOS but do not have any impact on line speed. Please note that the bandwidth command does not physically change the speed of an interface like the clock rate command does.


If no bandwidth command has been configured on the interface, Routing protocols and QOS will assume the line is running at the maximum speed supported by the interface which can result in incorrect routing and incorrect prioritization of packets. i.e. E1 line will have bandwidth of 2048 kbps in this case, no matter what is the clockrate set at DCE end.  To remedy this, we put the bandwidth command on DTE side(customer) of the interface to inform things like routing protocols what the REAL speed of the line is so its path calculations are correct. In general condition, clockrate and bandwidth should match.

Remember it is only the clock rate that will determine the speed of the line, not the bandwidth command. So if the clock rate has been set to 2.048Mbps, that is the speed the line will run at regardless of the bandwidth command.


 In another example, if you dont manually change the bandwidth value on the 64K serial line, the router will treat both as T1 lines. 


In real environment your service provider sets the speed of the line based on the subscription you have purchased with clockrate command.If the command is not used the interface will run at the maximum speed supported by the interface. If you have only subscribed for a 64k line, then the clock rate would have been set on the DCE side of the cable using the command ‘clock rate 64000’ under the interface. If you try to use the clock rate command on the DTE interface you will receive the error message "This command applies only to DCE interfaces". It is not possible to set the clock rate on the DTE side of the cable if everything is cabled correctly and you are using a real device (not a simulator or emulator like Boson or Dynamips/GNS3).


The Clock rate command, will define the speed and set the line to only operate at that speed. This is usually done by the telco, but obviously in a lap environment we would need to set one side of the cable as the DCE and hence set the clock rate of that line to dupe(fool) the other end(DTE).Show controller <interface name> command can be used to identify/check if your side is DTE or DCE.


DCE end


Router1#config t
Router1(config)# int serial1/2
Router1(config-if)# clock  rate 64000  ====>> represent 64 kbps


DTE end


device1#config t
device1(config)# int serial1/0
device1(config-if)# bandwidth 64  ====>> represent 64 kbps




In summary we simply need to understand two things:

Clock rate WILL set the speed. Whatever is defined with the clock rate command will force the line to run at the given speed.


Bandwidth is an indication to products like routing protocols and QOS as to what the line is supposed to be running at(used to identify routing preference based on cost)


Please note most of the routing protocols use bandwidth to calculate its metric(cost).