The bandwidth command sets an informational parameter to communicate only the current bandwidth to the higher-level protocols; you cannot adjust the actual bandwidth of an interface using this command.
This is only a routing parameter. It does not affect the physical interface.
For some media, such as Ethernet, the bandwidth is fixed; for other media, such as serial lines, you can change the actual bandwidth by adjusting the hardware. For both classes of media, you can use the bandwidth command to communicate the current bandwidth to the higher-level protocols.
Before the introduction of the bandwidth inherit command option, when the bandwidth value was changed on the main interface, the existing subinterfaces did not inherit the bandwidth value. If the subinterface was created before the bandwidth was changed on the main interface, the subinterface would receive the default bandwidth of the main interface, and not the configured bandwidth. Additionally, if the router was subsequently reloaded, the bandwidth of the subinterface would then change to the bandwidth configured on the main interface.
The bandwidth inherit command controls how a subinterface inherits the bandwidth of its main interface. This functionality eliminates inconsistencies related to whether the router has been reloaded and what the order was in entering the commands.
The no bandwidth inherit command enables all subinterfaces to inherit the default bandwidth of the main interface, regardless of the configured bandwidth. If the bandwidth inherit command is used without configuring a bandwidth on a subinterface, all subinterfaces will inherit the current bandwidth of the main interface. If you configure a new bandwidth on the main interface, all subinterfaces will use this new value.
If you do not configure a bandwidth on the subinterface and you configure the bandwidth inherit kbps command on the main interface, the subinterfaces will inherit the specified bandwidth.
In all cases, if an explicit bandwidth setting is configured on an interface, the interface will use that setting, regardless of whether the bandwidth inheritance setting is in effect.
Some interfaces (such as Asymmetric Digital Subscriber Line (ADSL), V.35, RS-449, and High-Speed Serial Interface (HSSI)) can operate with different transmit and receive bandwidths. The bandwidth receive command permits this type of asymmetric operation. For example, for ADSL, the lower layer detects the two bandwidth values and configures the Integrated Data Base (IDB) accordingly. Other interface drivers, particularly serial interface cards on low- and midrange-platforms, can operate in this asymmetric bandwidth mode but cannot measure their clock rates. In these cases, administrative configuration is necessary for asymmetric operations.