Clock rate versus bandwidth

If you have ever setup a lab of Cisco routers, you likely have configured a clock rate on one or more serial interfaces. You might have also configured the bandwidth setting on an interface as well. Is one required and not the other? Do they both need to be configured? That's what many people have wondered, including myself when I first started configuring Cisco routers over a decade ago. So I figured I'd create a quick explanation to help people better understand the bandwidth and clock rate commands.

Bandwidth

The bandwidth command administratively sets the bandwidth of an interface. When a router boots up, it sets a default bandwidth on an interface which can be seen with the show interface command.

R1#show int s0/0
Serial0/1 is administratively down, line protocol is down 
  Hardware is M4T
  MTU 1500 bytes, BW 1544 Kbit/sec, DLY 20000 usec,

[output omitted] 

The following example shows the bandwidth command syntax as well as the show interface output after the bandwidth has been changed:

R1#config t
R1(config)#int s0/0
R1(config-if)#bandwidth ?
  <1-10000000>  Bandwidth in kilobits
  inherit       Specify that bandwidth is inherited
  receive       Specify receive-side bandwidth

R1(config-if)#bandwidth 256
R1(config-if)#do show int s0/0
Serial0/0 is up, line protocol is up 
  Hardware is M4T
  Internet address is 192.168.1.1/30
  MTU 1500 bytes, BW 256 Kbit/sec, DLY 20000 usec, 

If the bandwidth is configured to 256Kbps, it doesn't mean that the interface only sends data at that rate. The router will still use 1.544Mbps (assuming a point to point T1). The bandwidth command is used for higher level protocols such as routing protocols and Quality of Service. If you configure EIGRP or OSPF, they will use the default bandwidth that is on an interface. If you manually configure the bandwidth on an interface the routing protocol would use that instead. 

It is common to use this command on interfaces connected to leased lines or connections provided by a service provider. For example, if you have a Metropolitan Ethernet connection that comes in on a Gigabit Ethernet interface, but the service provider has only provisioned it to 50Mbps, then using the bandwidth command to set it to 50,000 kbps would allow the router to accurately calculate the bandwidth on that link.

Clock Rate

The clock rate command is used to configure the rate at which bits of data will traverse the actual hardware interface. This command does impact the speed at which data traverses an interface.

The syntax for this command is as follows: 

R1(config-if)#clock rate ?
  With the exception of the following standard values not subject to rounding,

          1200 2400 4800 9600 14400 19200 28800 38400 
          56000 64000 128000 2015232 

  accepted clockrates will be bestfitted (rounded) to the nearest value
  supportable by the hardware.

  <246-8064000>    DCE clock rate (bits per second)

Typically when dealing with leased lines such as T1's from a service provider, you don't need to configure a clock rate because you are on the DTE side of the link. However, this command is frequently used in lab environments since one side of the serial connection is the DCE. 

For additional information information on the syntax of these commands, here is a link to the command reference at cisco.com

(http://www.tekcert.com)