Solutions for Exercises of Lecture 5 - Bridges

Transcription

1 Question 1: Solutions for Exercises of Lecture 5 - Bridges For the two hosts shown in the figure, the ethernet MAC addresses are provided. All the LANs are BRIDGE 7 LAN G LAN A LAN B BRIDGE 3 LAN C Host II port 4 BRIDGE 1 BRIDGE 2 LAN F 0:1:87:3e:ff:75 LAN D BRIDGE 6 BRIDGE 5 LAN E LAN H BRIDGE 4 LAN I Host I 5:80:87:3e:ff:75 ethernet LANs. Assume that the root path costs are computed using a hop count from the root (in other words, use the root path cost computation scheme shown in the lecture notes of lecture 5). Also assume that the bridge IDs are the numbers shown in the figure, i.e., the bridge ID of bridge 1 is 1, and so on. Answer the following questions: a. What BPDUs are generated by each bridge? Assume that all the bridges are operating Time Bridge 1 Bridge 2 Bridge 3 Bridge 4 Bridge 5 Bridge 6 Bridge 7 t=0 [1,0,1] [2, 0, 2] [3,0,3] [4,0,4] [5,0,5] [6,0,6] [7,0,7] t=1 [1,0,1] [1,1,2] [1,1,3] [2,1,4] [1,1,5] [1,1,6] [2,1,7] t=2 [1,0,1] [1,1,2] [1,1,3] [1,2,4] [1,1,5] [1,1,6] [1,2,7] in sync generating BPDUs based on the BPDUs received in the previous time slot. List the root ID, root path cost and sender s bridge ID using the three-tuple notation we used in the lecture notes. b. What is the root bridge? Bridge 1 1

2 c. What is the root port and root path costs of all the bridges? Bridge Root port Root path cost Bridge 1-0 Bridge Bridge Bridge Bridge Bridge Bridge d. What is the designated bridge and designated port for each LAN? LAN Designated bridge Designated port LAN A 2 1 LAN B 3 1 LAN C 1 1 LAN D 1 3 LAN E 5 1 LAN F 1 2 LAN G 2 2 LAN H 6 3 LAN I 5 2 For LAN H, bridge 4 is at a cost of 2 away from root, hence choose bridge 6; similarly, for lan I, bridge 5 is closer to the root than bridge 4. e. Which ports of each bridge are in forwarding and which ports are in blocking? Bridge forwarding blocking Bridge 1 1,2,3-2

3 Bridge forwarding blocking Bridge 2 1,2,4 3 Bridge 3 1,2 3 Bridge 4 1 2, 3 Bridge 5 1,2,3 Bridge 6 1,3 2 Bridge f. Draw a spanning tree with the root bridge as the root node of the tree, and show all other bridges and LANs in the tree. LAN F Bridge 1 LAN D LAN C Bridge 2 Bridge 5 Bridge 3 Bridge 6 LAN G LAN A Bridge 4 Bridge 7 LAN E LAN I LAN B LAN H g. When Host I sends a MAC frame to Host II, what is the total number of frames generated if the bridges have no addresses in their filtering databases (i.e., the bridges have not yet gone through their address learning phase)? Answer: 9 frames h. When Host I sends a MAC frame to Host II, what is the total number of frames generated if the bridges have all learned the location of host II, i.e., every bridge has a correct entry in its filtering database for host II? Answer: 2 frames Bridge 6 will forward it to. Bridge 4 receives it on a blocking port and hence will not forward. Bridges 1 and 3 will not forward since they have learnt where host II is located. i. For this case described in question (g), show the filtering database entry (MAC address and outgoing port number) for host II in each bridge. 3

4 The answer to this is none, because in question (g), nothing was learned and furthermore since host II does not send a frame, nothing can be learnt. Question 2: Solve the problem described on slide 37 in Lecture 5 notes. Make sure that you explain your answers. Root = 3 (lowest bridge ID) Root Path Cost = 0 (Bridge 3 itself is root) No root port. The configuration message is [3,0,3] All ports 1 to 5 are designated ports. Question 3: Solve the problem given on slide 38 in Lecture 5 notes. Assume that the bridge IDs are the numbers shown in the figure, i.e., the bridge ID of bridge 1 is 1, and so on. Show which messages are exchanged between the bridges until the algorithm is stabilized. BPDU sent by bridges: Table 1: BPDUs sent Time Bridge 1 Bridge 2 Bridge 3 Bridge 4 Bridge5 1 [1,0,1] [2,0,2] [3,0,3] [4,0,4] [5,0,5] 2 [1,0,1] [1,1,2] [1,1,3] [1,1,4] [1,1,5] Root ports: 2a, 3a, 4a, 5b Designated ports: 1a, 1b, 2b, 2c, 3b Blocking ports: 5a, 4b The spanning tree includes all the root ports and designated ports. Question 4: 4

5 Assume that the bridged network shown below executes the spanning tree algorithm. The Bridge IDs and BRIDGE 1 LAN G LAN A LAN B BRIDGE 2 port 4 BRIDGE 3 LAN C Host II LAN F 0:1:87:3e:ff:75 LAN D BRIDGE 4 BRIDGE 6 LAN E LAN H BRIDGE 5 LAN I Host I 5:80:87:3e:ff:75 port IDs are shown for all the bridges in the network. For the two hosts shown in the figure, the ethernet MAC addresses are provided. All the LANs are ethernet LANs. Assume that the root path costs are computed using a hop count from the root (in other words, use the root path cost computation scheme shown in the lecture notes of lecture 5). Answer the following questions: a. What BPDUs are generated by each bridge? Assume that all the bridges are operating Time Bridge 1 Bridge 2 Bridge 3 Bridge 4 Bridge 5 Bridge 6 t=0 [1, 0, 1] [2, 0, 2] [3, 0, 3] [4, 0, 4] [5, 0, 5] [6, 0, 6] t=1 [1, 0, 1] [1, 1, 2] [1, 1, 3] [3, 1, 4] [3, 1, 5] [2, 1, 6] t=2 [1, 0, 1] [1, 1, 2] [1, 1, 3] [1, 2, 4] [1, 2, 5] [1, 2, 6] in sync generating BPDUs based on the BPDUs received in the previous time slot. List the root ID, root path cost and sender s bridge ID using the three-tuple notation we used in the lecture notes. b. What is the root bridge? BRIDGE 1 5

6 c. What is the root port and root path costs of all the bridges? Bridge Root port Root path cost Bridge 1-0 Bridge Bridge Bridge Bridge Bridge d. What is the designated bridge and designated port for each LAN? LAN Designated bridge Designated port LAN A 1 2 LAN B 1 1 LAN C 2 2 LAN D 2 3 LAN E 4 1 LAN F 3 3 LAN G 3 2 LAN H 5 2 LAN I 4 2 e. Which ports of each bridge are in forwarding and which ports are in blocking? Bridge forwarding blocking Bridge 1 1, 2 Bridge 2 1, 2, 3 Bridge 3 1, 2, 3 4 Bridge 4 1, 2, 3 6

7 Bridge forwarding blocking Bridge 5 1, 2 3 Bridge 6 1 2,3 f. When Host I sends a MAC frame to Host II, what is the total number of frames generated (i.e., how many LANs see the frame) if the bridges have no addresses in their filtering databases (i.e., the bridges have not yet gone through their address learning phase)? Answer: 9 frames Bridges haven t learned addresses, MAC frames are generated on lan h, through bridge 5 to lan g, through bridge 3 to lan f and lan a, through bridge 4 to lan e, lan i, through bridge 1 to lan b, through bridge 2 to lan c, lan d: total of 9 MAC frames g. When Host I sends a MAC frame to Host II, what is the total number of frames generated (i.e., how many LANs see the frame) if the bridges have all learned the location of host II, i.e., every bridge has a correct entry in its filtering database for host II? Answer: 5 frames lan h, through bridge 5 to lan g, through bridge 3 to lan a, through bridge 1 to lan b, through bridge 2 to lan d: 5 MAC frames h. For this case described in question (g), show the filtering database entry (MAC address and outgoing port number) for host II in each bridge. Bridge MAC address Outgoing port number Bridge 1 0:1:87:3e:ff:75 1 Bridge 2 0:1:87:3e:ff:75 3 Bridge 3 0:1:87:3e:ff:75 1 Bridge 4 0:1:87:3e:ff:75 3 Bridge 5 0:1:87:3e:ff:75 1 Bridge 6 0:1:87:3e:ff:75 1 Hint: Remember that if a bridge does not have an entry for a MAC frame in its filtering database, it sends the MAC frame on its ports that are on the spanning tree (of course excluding the port on which it received the frame). 7

8 i. Draw the spanning tree: Bridge 2 lan b lan d Bridge 1 Bridge 4 lan c lan f lan e lan a lan i Bridge 3 lan g Bridge 5 lan h Bridge 6 8