If the capacity of the interface is exceeded, the frame that is currently being received is dropped and the overrun counter is incremented.
Each network interface on Cisco routers consists of a chipset for converting signals received in the media into bits of information, and a small packet buffer into which this information is stored before being copied into the I/O memory. On some interface types, this chipset and packet buffer cannot handle a long burst of frames. Such interfaces are meant to provide connectivity to a certain network type, and not to switch packets at line rate. The line rate of these interfaces is often higher than the switching capacity of the router. Therefore, building an interface that receives more traffic than the router can handle only increases the cost, without adding any real value to the router architecture.
Some examples of connectivity interfaces include PA-GE on c7200 routers, GEIP on c7500 routers, and FastEthernet interfaces on c3620 and c3640 routers.
In a small number of cases, the overrun counter may be incremented because of a software defect. However, in the majority of cases, it indicates that the receiving capability of the interface was exceeded. Nothing can be done on the router that reports overruns. If possible, the rate that frames are coming should be controlled at the remote end of the connection. Otherwise, if the number of overruns is high, the hardware should be upgraded.
Note: If the router drops packets because of performance problems of the main CPU (and not because of the capacity of the interface itself), the ignored counter is increased, and not the overruns counter.