bandwidth [kbps]
Sets the bandwidth value (in kbps) on an interface for routing protocol metric calculations and QoS, overriding the default detected bandwidth.
bandwidth [kbps]When to Use This Command
- Adjusting OSPF or EIGRP cost on a serial link to influence path selection
- Simulating lower-speed link for testing routing protocol behavior
- Setting bandwidth on a tunnel interface to match the underlying path capacity
- Configuring QoS bandwidth parameters on an interface
Command Examples
Set bandwidth on a serial interface
Router(config-if)# bandwidth 128Router(config-if)#
No output is displayed upon successful configuration. The command sets the interface bandwidth to 128 kbps.
Verify bandwidth setting with show interface
show interfaces serial 0/0/0Serial0/0/0 is up, line protocol is up
Hardware is HD64570
Internet address is 192.168.1.1/30
MTU 1500 bytes, BW 128 Kbit/sec, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set
Keepalive set (10 sec)
...The 'BW 128 Kbit/sec' line shows the configured bandwidth. This value is used by routing protocols (e.g., OSPF cost = 10^8 / BW) and QoS.
Understanding the Output
The 'bandwidth' command itself produces no output. To verify the setting, use 'show interfaces' or 'show ip ospf interface'. In 'show interfaces', look for 'BW' followed by the value in Kbit/sec. This value affects OSPF cost (cost = reference bandwidth / interface bandwidth), EIGRP composite metric (bandwidth is a factor), and QoS bandwidth allocation. A common mistake is confusing bandwidth with actual link speed; this command only affects routing metrics and QoS, not the physical speed.
CCNA Exam Tips
CCNA exam tip: OSPF cost is calculated as 10^8 / bandwidth (in bps). Changing bandwidth changes cost and thus path selection.
CCNA exam tip: EIGRP uses bandwidth in its metric formula; a lower bandwidth increases the metric.
CCNA exam tip: The 'bandwidth' command does not change the actual line speed; it only affects routing protocol calculations.
CCNA exam tip: On Ethernet interfaces, default bandwidth is 10,000 kbps for FastEthernet and 1,000,000 kbps for GigabitEthernet.
Common Mistakes
Mistake 1: Setting bandwidth to a value higher than the actual link speed, causing routing protocols to prefer this link incorrectly.
Mistake 2: Forgetting that the bandwidth command is in kbps, not bps; e.g., setting 'bandwidth 1000000' for 1 Gbps instead of 'bandwidth 1000000' (which is correct for 1 Gbps = 1,000,000 kbps).
Mistake 3: Confusing 'bandwidth' with 'speed' command; speed sets the actual interface speed, while bandwidth only affects metrics and QoS.
Related Commands
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
show ip eigrp topology
Displays the EIGRP topology table, showing all learned routes and their feasible successors, used to verify EIGRP convergence and path selection.
show ip ospf interface
Displays OSPF interface parameters and status, used to verify OSPF configuration and troubleshoot neighbor relationships.
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