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).
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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.
4.6 Which bridge in a bridged LAN will become the “root bridge” of the spanning tree ?
A. (a) The bridge selected randomly
B. (b) The bridge with the largest bridge ID
C. (c) The bridge with the smallest bridge ID
D. (d) The bridge in the “center” of the bridged LAN
4.7 What is the “bridge ID” of a bridge in a bridged LAN ?
A. (a)6 bytes, MAC address of the bridge
B. (b)8 bytes, priority (2 bytes) + MAC address of the bridge (6 bytes)
C. (c)10 bytes, IP address (4 bytes) + MAC address of the bridge (6 bytes)
D. (d)6 bytes, priority (2 bytes) + IP address of the bridge (4 bytes)
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