logic. With cut-through processing, the first bits of the frame are sent out the outbound port
before the last bit of the incoming frame is received instead of waiting for the entire frame to
be received. In other words, as soon as the switching port receives enough of the frame to see
the destination MAC address, the frame is transmitted out the appropriate outgoing port to the
destination device. The unfortunate side effect is that because the frame check sequence (FCS)
is in the Ethernet trailer, the forwarded frame may have bit errors that the switch would have
noticed with store-and-forward logic. And, of course, if the outbound port is busy, the switch
will store the frame until the port is available.
regardless, the internal processing can be categorized as one of the methods listed in Table 4-6.
sending a frame to Wilma, and Barney sending a frame to Betty.
the frame coming in Port 2 and out Port 4. These frames also are in four different collision
domains. For these reasons, no collision occurs. A four-port transparent bridge would behave
the same way, but switches are optimized for concurrent frame forwarding, so latency is likely
to be less with a switch.
server (Pebbles) that is both sending and receiving a frame at the same time. Betty and Wilma
are in different collision domains, so Pebbles cannot undergo a collision due to the nature of
full-duplex Ethernet.
frame (forward). This allows the switch to check the FCS before forwarding
the frame. (FCS is in the Ethernet trailer.)
address field in the header is received. The first bits in the frame can be sent
out the outbound port before the final bits in the incoming frame are
received. This does not allow the switch to discard frames that fail the FCS
check. (FCS is in the Ethernet trailer.)
received before forwarding the first bytes of the outgoing frame. According
to Ethernet specifications, collisions should be detected during the first 64
bytes of the frame; frames in error due to collision will not be forwarded.
The FCS still cannot be checked.