Which switching method always checks for CRC errors before forwarding the frame? (Select the best answer.)

A. storeandforward

B. cutthrough
C. adaptive cutthrough
D. FragmentFree

Explanation:

The store-and-forward switching method always checks for cyclic redundancy check (CRC) errors before forwarding the frame, thereby ensuring reliability. When the store-and-forward switching method is used, the switch receives the entire frame before forwarding the frame. By receiving the entire frame, the switch can verify that no CRC errors are present in the frame; this helps prevent the forwarding of frames with errors. If a CRC error is detected, the frame is discarded. Similarly, the frame is discarded if it contains fewer than 64 bytes; a frame containing fewer than 64 bytes is referred to as a runt. Because the entire frame must be received before it can be forwarded, the store-and-forward switching method has a higher latency than other switching methods. Multilayer switches must use the store-and-forward switching method, because the switch must receive the entire frame before Network layer operations can be performed.

In contrast to the store-and-forward switching method, the cutthrough switching method begins forwarding a frame as soon as the frame’s destination address is received. Thus the switch begins forwarding the frame before the frame is fully received, which helps reduce latency. However, with the cutthrough method, the frame is not checked for errors prior to being forwarded.

The adaptive cutthrough switching method provides a balance between store-and-forwardswitching and cutthrough switching. When adaptive cutthrough switching is used, an error threshold can be configured that enables the switch to take advantage of the low latency provided by cutthrough switching until the configured error threshold is reached? at that point, the switch begins using the higher latency store-and-forward switching method. When the error rate falls below the configured threshold, the switch reverts to using the cutthrough switching method.

The FragmentFree switching method is similar to cutthrough switching in that the switch does not have to receive the entire frame before it begins forwarding the frame. However, unlike cutthrough switching, FragmentFree switching waits until at least 64 bytes of the frame have been received before deciding whether to forward the frame. This allows the switch to check for collision fragments, which should be detectable within the first 64 bytes of the frame. If the frame is not a collision fragment, the switch begins forwarding the frame towardthe destination.

Reference:
Cisco: LAN Switching and VLANs: LAN Switching ForwardingCisco: Configuring Switching Modes