spanning tree exercise 4.1 Assume a bridge receives a frame from - PDF document

spanning tree exercise 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)The bridge will broadcast the frame to all ports if D is not found in the Filtering database of the bridge. (b)The bridge will broadcast the frame to all ports, except port x, if D is not found in the Filtering database. (c)The bridge will filter the frame if D is found in the Filtering database and D belongs to port x. (d)The bridge will forward the frame to port y, if D is found in the Filtering database and D belongs to port y. ANS: (a) 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)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)The bridge will new entry of R = (D,x,0) to the Filtering database if D is not found in the Filtering database. (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)If an entry R = (S, x, 20) already existed in the Filtering database, then the bridge will update the entry to (S, x, 0). ANS: (a)(c)(d) 4.3 Consider the following BLAN configuration. Initially, assume the Filtering databases are empty. Fill in each entry (an integer to present a port number) in the following table after the following five transmissions. Use integer “ 0 ” to indicate if a station is not learned in the corresponding filtering database.

(1) Station A transmits a frame to station B (2) Station B transmits a frame to station C (3) Station D transmits a frame to station A (4) Station E transmits a frame to station D (5) Station C transmits a frame to station B < Port number of Filtering Port number of Filtering Port number of Filtering Station database X database Y database Z 1 2 1 A 2 2 1 B 2 1 0 C 2 3 1 D 0 3 2 E 4.4 Which of the following statements are correct ? (a)To provide a reliable network connectivity, many physical loops may exist in a bridged LAN (BLAN). (b)No physical loop is allowed in a bridge LAN. (c)Before transmitting frames, the spanning tree of a BLAN should be constructed first.

(d)Based on the spanning tree structure, there is only one communication path between each pair of bridges. (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). ANS: (a)(c)(d) 4.5 What may happen if loops exist in a bridged LAN and the spanning tree is not established ? (a)The receiver may receive duplicated frames. (b)The bridges may learn wrong ports for MAC addresses. (c)The frames will be filtered by bridges correctly. (d)Each frame will be delivered to the destination via the shortest path between the source and destination stations. ANS: (a)(b) 4.6 Which bridge in a bridged LAN will become the “root bridge” of the spanning tree ? (a)The bridge selected randomly (b)The bridge with the largest bridge ID (c)The bridge with the smallest bridge ID (d)The bridge in the “center” of the bridged LAN ANS: (c) 4.7 What is the “bridge ID” of a bridge in a bridged LAN ? (a)6 bytes, MAC address of the bridge (b)8 bytes, priority (2 bytes) + MAC address of the bridge (6 bytes) (c)10 bytes, IP address (4 bytes) + MAC address of the bridge (6 bytes (d)6 bytes, priority (2 bytes) + IP address of the bridge (4 bytes) ANS: (b)

4.8 Which of the following statements are correct about the ID of a bridge ? (a)The ID is fixed (b)The ID is programmable by setting MAC address (c)The ID is programmable by setting priority value (d)ID 1 > ID 2 if the MAC address of ID 1 > MAC address of ID 2. (e)ID 1 > ID 2 if the priority value of ID 1 > the priority value of ID 2. ANS: (c)(e) 4.9 The “root path cost” (RPC) of a b ridge is defined as the summation of the transmission costs of the bridges along the path from the bridge to the root bridge. Consider the following BLAN configuration, the RPC of bridge 2 is __(a)__, and the RPC of bridge 4 is __(b)__. 10 (a) 10 (b)

送出檢查 顯示答案 4.10 Let (A) be the procedure to find the root bridge of a BLAN, (B) be the procedure to find the “root port” of a bridge, and (C) be the procedure to find a “designated brid ge” of a LAN, (D) be the procedure to find a “designated port” of a LAN. What is the sequential procedure to establish the spanning tree of a BLAN ? (a) A, B, D (b) A, C, B (c) A, C, D (d) B, A, C ANS: (a) 4.11 Consider the following state diagram of a bridge port. Initially, a port is in the “blocked state”. The state of a port is changed to “listen” state when a port is selected as a “root” (R) port or a “designated” (D) port. The state of a port is changed to “block” state when a port is no more an R port or a D port. The state of a port is changed to “learning” state from “listen” state after a forwarding delay. The state of a port is changed to “forwarding” state from “learning” state after another forwarding delay. Which of the following statements are correct ? (a) Before the spanning tree is completely constructed, the state of a port may be changed among these four states. (b) After the spanning tree is constructed, the state of a port is either in the “learning state” or “forwarding state”.

(c) A port in the “forwarding state” will be an R port or a D port. (d) A port in the “learning state” state is able to learning the source MAC address of a received frame but unable to forward frames. (e) (e) A port in the “forwarding state” state is able to forward or filter the received frames. ANS: (a)(c)(d)(e) 4.12 Which of the following statements about the spanning tree maintenance are correct ? (a) The root bridge will send a “Hello” BPDU periodically to each of its ports. (b) The “Hello” BPDUs are delivered into each of the LANs along the paths of the spanning tree. (c) To have better reliability, each bridge will also forward the “Hello” BDPUs into each of its ports. (d) For the “root bridge”, all it’s ports are “designated ports”. ANS: (a)(b)(d) 4.13 To construct the spanning tree of a BLAN, initially, each bridge assume itself is the “root bridge”, set each of it’s ports as a “designated” port, and send a BPDU to each port with its bridge ID, root bridge ID (itself), and the root path cost of 0. Which of the following statements are correct when a bridge (with root bridge ID = w, rpc = p) receives a BPDU (with root bridge ID = m, rpc =q) from port x ? (a) port x will be selected as a new R port if w > m (b) port x will be selected as a new R port if w = m and p > q (c) port x will be selected as a new R port if p > q (d) port x will be selected as a new R port if w < m ANS: (a)

4.14 Consider the procedure to construct the spanning tree. When a bridge (with root bridge ID = w, rpc = p) receives a BPDU (with root bridge ID = m, rpc =q) from port x (with a transmission cost of c). Assume w > m. Then what will happen ? (a) Port x will be selected as a new R port. (b) port x will be selected as a new D port. (c) A BPDU with (with root bridge ID = m, rpc =q+c) will be forwarded to all the ports, except port x. (d) A BPDU with (with root bridge ID = m, rpc =q) will be forwarded to all the ports, except port x. (e) A BPDU with (with root bridge ID = m, rpc =p+q) will be forwarded to all the ports, except port x. ANS: (a)(c) 4.15 Consider the procedure to construct the spanning tree. When a bridge (with root bridge ID = w, rpc = p) receives a BPDU (with root bridge ID = m, rpc =q) from port x (with a transmission cost of c). Assume w < m. Then what will happen ? (a) Port x will be selected as a new R port. (b) The BPDU will be dropped directly. (c) A BPDU (with root bridge ID = w, rpc =p) will be forwarded to port x. (d) A BPDU (with root bridge ID = w, rpc =q+c) will be forwarded to port x. ANS: (b)(c) 4.16 Consider the procedure to construct the spanning tree. When a bridge (with root bridge ID = w, rpc = p) receives a BPDU (with root bridge ID = m, rpc =q) from port x (with a transmission cost of c). Assume w = m and p > q+c. Then what will happen ? (a) Port x will be selected as a new R port. (b) The BPDU will be dropped directly. (c) A BPDU with (with root bridge ID = m, rpc =q+c) will be forwarded to all the ports, except port x. (d) A BPDU (with root bridge ID = w, rpc =q+c) will be forwarded to port x.