What is the Difference between the Clock Rate and Bandwidth?

In this below post I covered difference between Clock Rate and Bandwidth on Serial interface in Cisco Router.

Clock Rate Command

The clockrate xxxx command (configured on the DCE side) is used to configure the actual speed of the serial line you configure it on. I mean clock rate decides the actual speed of the physical medium between you and your service provider.

In order to understand clock rate we first need to understand how the cabling works on Routers. When connecting two Routers together with a serial cable, one of the Routers needs to host the DCE (Data Communications Equipment) side of the cable, and the other will host the DTE (Date Terminal Equipment) side. Most serial cables are marked on the connector if it’s the DTE or DCE. In Cisco Routers you can find the status using show controllers serial <> command.

So what’s the difference between the DCE and DTE? The DCE is the master and this side that sets the speed of the link (clock rate). Based on this, it’s safe to assume that the cable coming out of your router and going to your service provider is the DTE side and it acts as slave, since you service provider sets the speed of the line based on the subscription you have purchased. The DTE side of the cable is where the communications terminate ie: your router terminates the connection from the service provider.

From a configuration point of view the DCE side of the cable is able to use the clock rate command to set the speed of the line. If the command is not used the interface will run at the maximum speed supported by the interface.

Clock Rate Configuration on Cisco Router Interface

If you have only subscribed for a 1Mbps line, then the clock rate would have been set on the DCE side of the cable using the command ‘clock rate 1024000’ under the interface.

Cisco-Router# conf t
Cisco-Router(config)# interface serial 0/0
Cisco-Router(config-if)# clock rate 1024000 (it represented in Bits per second)

If you have subscribed for only 128k line from your service provider, then the clock rate would have been set on the DCE side of the cable using the command ‘clock rate 128000’ under the interface.

Cisco-Router# conf t
Cisco-Router(config)# interface serial 0/0
Cisco-Router(config-if)# clock rate 128000 (it represented in Bits per second)

Bandwidth Command

Bandwidth Command is use to inform your device, what is speed of the line which is connected to the serial interface. Remember this command will not affect the actual speed of link, it’s just a method of informing your device what speed the link should run at. The reason the value has to be defined under serial interface is that the device will assume the link is running at default maximum speed of Serial interface if the command has not been configured. If the actual link speed is 1Mbps but your device assumes default speed of Serial interface 2Mbps , This will have a negative effect on things like routing protocols and QOS (Quality of Service).

The bandwidth command is just a metric used in various calculations for that interface. For instance the “load” in show interface Se0/x is calculated from the bandwidth parameter. IGP routing protocols also use the bandwidth parameter when calculating path-selection.

Bandwidth Configuration on Cisco Router Interface

Cisco-Router# conf t
Cisco-Router(config)# interface serial 0/0
Cisco-Router(config-if)# bandwidth 1024 (It represents in Kbps)

Cisco-Router# conf t
Cisco-Router(config)# interface serial 0/0
Cisco-Router(config-if)# bandwidth 128 (It represents in Kbps)