Clock rate and Bandwidth commands

Two commands in Cisco router configuration that often cause confusion are the clock rate and bandwidth commands. Let me explain the difference between these two important interface configuration commands.

Clock Rate Command

The clock rate command is used on serial interfaces to set the actual physical transmission speed of the link. This command is only effective on the DCE (Data Communications Equipment) side of a serial connection.

Key points about clock rate:

• Only works on DCE interfaces
• Sets the actual physical speed of the serial link
• Measured in bits per second (bps)
• Must match on both ends of the connection

Example configuration:

Router(config)# interface serial0/0
Router(config-if)# clock rate 64000

Bandwidth Command

The bandwidth command is used to inform routing protocols about the speed of an interface. This is a logical setting that doesn't change the actual physical speed of the link.

Key points about bandwidth:

• Used by routing protocols for metric calculations
• Does not change actual physical speed
• Measured in kilobits per second (kbps)
• Important for protocols like EIGRP and OSPF

Example configuration:

Router(config)# interface serial0/0
Router(config-if)# bandwidth 64

Key Differences

The main differences between these commands:

• Purpose: clock rate sets physical speed, bandwidth informs routing protocols
• Units: clock rate uses bps, bandwidth uses kbps
• Scope: clock rate only works on DCE, bandwidth works on any interface
• Effect: clock rate changes actual transmission speed, bandwidth only affects routing metrics

When to Use Each Command

Use clock rate when:

• Configuring a serial interface that acts as DCE
• You need to set the actual transmission speed
• Working with back-to-back serial connections in a lab

Use bandwidth when:

• You want routing protocols to make better path decisions
• The default bandwidth doesn't match the actual link speed
• Configuring QoS policies that depend on interface bandwidth

Understanding these two commands is essential for proper router configuration and network optimization.