Understanding the Actions Taken by a Cisco Ethernet Switch when Receiving Runt Frames

What happens to runt frames received by a Cisco Ethernet switch?

When a Cisco Ethernet switch receives a runt frame, which refers to an Ethernet frame that is shorter than the minimum frame length defined by the Ethernet standard (64 bytes without counting the preamble and delimiter), several actions can take place depending on the configuration of the switch.

1. If the switch is operating in cut-through mode: In this mode, the switch forwards frames as soon as the destination MAC address is identified, without fully receiving the entire frame. When a runt frame is received, the switch may still forward it to the appropriate interface before it has fully arrived. However, runt frames are typically discarded because they may indicate issues such as collisions or corrupted data.

2. If the switch is operating in store-and-forward mode: In this mode, the switch waits to receive the entire frame before forwarding it. When a runt frame is received, the switch examines the frame’s FCS (Frame Check Sequence) to detect errors. If the FCS indicates an error, the switch discards the runt frame. If no errors are detected, the switch may still process and forward the runt frame.

3. Depending on the configuration, a switch can be instructed to discard runt frames to avoid further transmission and limit network congestion caused by potentially corrupted or incomplete frames.

It’s worth noting that runt frames usually indicate problems in the network, such as collisions, faulty network interface cards, or misconfiguration. Therefore, it’s important to investigate and resolve the underlying cause of runt frame occurrences to ensure optimal network performance.

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