Which of the following are correct for traditional Ethernet CSMA/CD performance ?
A. (a) Ethernets work best under lightly loaded conditions.
B. (b) Ethernets work very well under heavy loads as all collisions will be detected.
C. (c) It’s a fair network that all nodes may share the bandwidth very equally.
D. (d) Under heavy loads, too much of the network’s capacity is wasted by collisions.
E. (e) The throughput of an Ethernet depends on the traffic load, heavy load will have better throughput.
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4.1 Assume a bridge receives a frame from port x, with destination MAC address D and source MAC address S. Which of the following statements about frame forwarding are NOT correct ?
A. (a) The bridge will broadcast the frame to all ports if D is not found in the Filtering database of the bridge.
B. (b) The bridge will broadcast the frame to all ports, except port x, if D is not found in the Filtering database.
C. (c) The bridge will filter the frame if D is found in the Filtering database and D belongs to port x.
D. (d) The bridge will forward the frame to port y, if D is found in the Filtering database and D belongs to port y.
4.2 Assume a bridge receives a frame from port x, with destination MAC address D and source MAC address S. Let R = (address, port, timer) be a record in the Filtering database, where timer (in seconds) is the duration this record existed (initially, timer = 0). Which of the following statements about address learning are correct ?
A. (a) The bridge will add a new entry of R = (S,x,0) to the Filtering database if S is not found in the Filtering database.
B. (b) The bridge will new entry of R = (D,x,0) to the Filtering database if D is not found in the Filtering database.
C. (c) If an entry R = (S, y, 20) already existed in the Filtering database, then the bridge will update the entry to (S, x, 0).
D. (d) If an entry R = (S, x, 20) already existed in the Filtering database, then the bridge will update the entry to (S, x, 0).
4.4 Which of the following statements are correct ?
A. (a) To provide a reliable network connectivity, many physical loops may exist in a bridged LAN (BLAN).
B. (b) No physical loop is allowed in a bridge LAN.
C. (c) Before transmitting frames, the spanning tree of a BLAN should be constructed first.
D. (d) Based on the spanning tree structure, there is only one communication path between each pair of bridges.
E. (e) For the spanning tree constructed by the IEEE 802.1D spanning tree algorithm, the communication path between each pair of bridges is optimized (shortest).
4.5 What may happen if loops exist in a bridged LAN and the spanning tree is not established ?
A. (a) The receiver may receive duplicated frames.
B. (b) The bridges may learn wrong ports for MAC addresses.
C. (c) The frames will be filtered by bridges correctly.
D. (d) Each frame will be delivered to the destination via the shortest path between the source and destination stations.
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