Clock rate and Bandwidth commands
Two commands which are often confused or misunderstood, are the clock rate and bandwidth commands.
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.
So what’s the difference between the 2 sides? The DCE side of the cable is the side that sets the speed of the link (also known as clocking). 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, 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. 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.
AOIP.ORG# conf t
AOIP.ORG(config)# interface serial 1/0
AOIP.ORG(config-if)# clock rate 64000 (represented in BITS per second)
If you try use the clock rate command on the DTE interface you will receive the following error message “This command applies only to DCE interfaces”
To identify which end of the cable has been plugged into a Cisco router, you can also use the command “show controller” – see below diagram.
The bandwidth command however does not adjust the speed of the line at all, however it should be configured on ALL DTE and DCE interfaces because it is used by;
1/ Routing protocols – to calculate the cost of a path
2/ QOS (Quality of Service) – to identify how much bandwidth is availble to prioritize,
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.
The bandwidth command is issued under the interface, as is represented in Kilobits per second
AOIP.ORG# conf t
AOIP.ORG(config)# interface serial 1/0
AOIP.ORG(config-if)# bandwidth 64 (represented in kilobits per second)
