## COIT20261 Network Routing and Switching Assignment

Assessment item 2—Written Assessment

Question 1 Routing Table construction                                           (10 marks)

Consider a network scenario with four routers (labelled R1, R2…) and six subnets. A default router has been configured and all routers are fully updated. This scenario is shown in the below graphic. Your task: using the table blanks provided, supply all the details of the routing table for routers R1 and R2. Arrange in longest-mask order, supply masks in dotted-decimal format.

Routing table of router R1

Routing table of router R2

Question 2 – Forwarding process – classless addressing                           (8 marks)

First study and understand the forwarding process as explained in the lecture (based on Forouzan). Answer the following two questions.

1. Refer to Figure 1 network diagram from Question 1. Assume that a packet arrives at router R3 with a destination address of 140.22.0.2. Explain the router’s logical processing of this packet (show its calculations as well) and its forwarding decision. (1 mark for correct forwarding decision, 3 marks for clear and correct explanation including calculations).
2. In the same diagram (Figure 1), assume that a packet arrives at router R4 with a destination address of 161.22.14.10. Explain the router’s forwarding decision and the processes it used to reach that decision. (1 mark for correct forwarding decision, 3 marks for clear and correct explanation including calculations).

Question 3 – Distance Vector 3-step routing algorithm                                         (8 marks)

In Week 8, we discuss the algorithm used to calculate shortest path using RIP. We used this diagram as an aid:

Working through this question will give you a feel for the logical processing by which Routers update each other by broadcasting a copy of their table to one another. You may find reviewing the materials for Week 8 helpful (e.g. the diagrams). Draw the network diagram described by the tables below (use the slides in week 8 as a guide) and update the diagram as you go – this will help you answer the questions or verify your answers.

Shown below are the initial state routing tables for Routers A, B, C, D and E which all have the same destination networks listed, abbreviated to destinations A, B, C, D and E, with aggregated costs. For example, a cost of 6 equates to 6 hops. The solution table blanks are provided in the Answer Template. The initial state tables (five of them) are shown below:

Router A’s table                                       Router B’s table                                        Router C’s table

To = Available network destinations

Cost = The number of hops to reach the network

Next = The next hop toward the destination

∞ = infinity (network unknown, cannot be reached)

1. Draw a diagram to show the topology of the network (attach a screenshot to show your answer). (2 marks)
• Router D receives an update from Router C and processes it, displaying Router D’s modified and new tables. (1.5 marks)

• After this, Router E receives an update from Router A and processes it, displaying Router E’s modified and new tables. (1.5 marks)

• Convergence is achieved when all routers in a routing domain finish updating their routing information and reach a consensus on network reachability. Write each router’s final table when the convergence is achieved. (3 marks)

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Question 4 Fragmentation                                                                            (4 marks)

A router receives an IP datagram with a total length of 2,400 bytes and no options. The router determines that the next destination network has a maximum transmission unit (MTU) limit of 1,000 bytes, which the datagram must adhere to during transmission. Answer the following questions, showing your calculations and reasoning.

1. Suppose the router decides to fragment the packet into three fragments, with each fragmented packet containing as much payload as possible except for the final fragment. Calculate the possible total lengths and payload lengths of the three new segmented packets. (1.5 marks)
2. Compute the fragmentation offset, DF (Don’t Fragment), and MF (More Fragments) flags for every fragment. (1.5 marks)
3. The total size of the three fragments leaving the router will be greater than the size of the original datagram that arrived. Please explain why and by how much. (1 mark).