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F. Stored and forwarded.
Answer: F
Explanation: In store and forward mode, the switch must receive the complete frame before forwarding takes
place. The destination and source addresses are read, the cyclic redundancy check (CRC) is performed, relevant
filters are applied, and the frame is forwarded. If the CRC is bad, the frame is discarded. The latency (or delay)
through the switch varies with frame length.
Incorrect Answers:
A is incorrect; CSMA/CD is the physical method used on a 802.3 LAN
B is incorrect; Full duplex is when you can transmit and receive at the same time.
C is incorrect; in cut-through mode the frame is forwarded after the Destination Address has been read.
D is incorrect; Half Duplex means EITHER transmit or receive, but not both at the same time,
E is incorrect; this is not a switch mode. Fragment-free mode is a switch mode.
Steve McQuerry. Interconnecting Cisco Network Devices. (Cisco Press: 2000) pages 162-163.
Q. 123
Switching methods include cut-through, store and forwarded and a modified version of the first two
methods. Which statement about switching methods is true?
A. The stored and forward method has low latency.
B. The cut through method and switching has high latency.
C. The modified version holds the packet in memory until 50% of the packet reaches the switch.
D. The modified version holds the packet in memory until the data portion of the packet reaches the
switch.
Answer: D
Explanation: The modified cut-through (also known as fragment-free) the switch reads into the first 64 bytes
before forwarding the frame.
Incorrect Answers:
A is incorrect; Store and Forward has HIGH latency.
B is incorrect; Cut-Through has LOW latency.
C is incorrect; it is not 50%. In the modified version, the frame transmission does not start until the first 64
bytes are read.
Steve McQuerry. Interconnecting Cisco Network Devices. (Cisco Press: 2000) pages 162-163.