When an ethernet frame arrives at a Cisco switch, the switch will determine the size of the frame and determine whether the frame is a runt frame. If the frame is determined to be a runt frame, then the switch will discard the frame.
A Cisco switch receives frames received by one of its ports. The switch forwards the frames to the other ports. There are two types of frames. These are unicast and multicast frames. Unicast frames carry data to one destination while multicast frames deliver the data to more than one destination. Multicast frames are sent through the network. This frame is not forwarded.
How to prevent runt frames from being created:
The most common cause of runt frames on Cisco IOS switches is due to an issue with the FCS-RD frame generation process. This process takes a certain number of bits to generate a frame and the number of bits is related to the MTU size (maximum transmission unit).
If the packet size is smaller than the MTU size (or number of bits needed for the frame) then the packet will be dropped. This occurs because the FCS calculation is done before the MAC header, and the MAC header is dependent on the packet size (MAC header has a fixed size).
To resolve this, you need to increase the MTU size of the interface, which can be done using the command
mtu mtu-value [interface name]
where mtu-value is an integer that represents the maximum packet size allowed on the interface.
If the receive port is configured as a receive only switch port and the interface is a non-bridge interface or trunk port (included in the configuration of the interface) with a unicast addressing scheme, then the receive port cannot be used as a source port for the frame.
If the receive port is configured as a receive only switch port and the interface is a bridge interface with a unicast addressing scheme, then the receive port cannot be used as a source port for the frame.
For more information about receive port configuration, see the Cisco Catalyst IP Services Configuration Guide.
In the end, runt frames can have a significant impact on network performance and should be avoided if possible. Ethernet switches will typically drop any runt frame that is received, but this can cause further problems if the frame is actually needed by the network. There are a few ways to prevent runt frames from being created in the first place, and it’s important to understand these methods and use them where appropriate.
