Cut-through switching behavior when buffering packets (VLAN priority, Error packets)

[Frederic Depuydt]

Hi,

How does a cut-through switch behave when buffering packets? In order to situate my question I have added the following paragraph of wikipedia:

There are certain scenarios that force a cut-through Ethernet switch to buffer the entire packet:

• Speed: When the outgoing port is faster than the incoming port, the switch must buffer the entire frame received from the lower-speed port before the switch can start transmitting that frame out the high-speed port, to prevent underrun. (When the outgoing port is slower than the incoming port, the switch can perform cut-through switching and start transmitting that frame before it is entirely received, although it must still buffer some of the frame).
• Congestion: When a cut-through switch decides a frame from one incoming port needs to go out through an outgoing port, but that outgoing port is already busy sending a frame from a second incoming port, the switch must buffer some or all of the frame from the first incoming port.

Does a cut-through switch completely act as a store-and-forward switch for the packets in these cases? Meaning, removing error packets from the buffer and giving priority to packets with higher VLAN priority?

Kind regards,

Frederic Depuydt

[Joseph W. Doherty]

The answer likely depends on the particular architecture of the device and the level of detail needed, to answer your question, might be considered proprietary.

For example, consider an egress 10g and an ingress gig and 10g. A frame begins to arrive on the gig interface. We need to buffer it. While the gig frame is being buffered, a frame arrives on the 10g ingress interface. Assuming the 10g egress is free, do we transmit it, or wait on the gig frame which arrived first? In theory, you might be able to fully transmit the 10g frame, especially with cut-through, while still waiting on the gig frame to be fully received.