The Bandwidth can be limited on the serial ports of a router using the bandwidth command. However, this feature is not available in a switch and hence the bandwitdth cannot be restricted although the bandwidth command might still be available in the cli. If the port is a fast ethernet port, you have a choice between 10 Mbps and 100 Mbps, but you cannot choose 20 Mbps. You could configure the switch to use 10 Mbps instead.
the bandwith configuration on a serial routerinterface does _not_ affect the traffic flow-rate itself. It's only for some routing protocols to calculate the best path (OSPF). Additionaly it is used for some QoS features (like CAR ...), but it has no traffic limiting capability itself.
