Consider the 3 LANs interconnected by 2 routers as shown in Figure 2 below. c. C

Question

Consider the 3 LANs interconnected by 2 routers as shown in Figure 2 below.

c. Consider sending an IP datagram from host F to host E. Will host E ask router R1 to help forward the datagram? Why? In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses?
d. Suppose F would like to send an IP datagram to A, and assume that F’s ARP cache does not contain A’s MAC address. Will F perform an ARP query to find A’s MAC address? Why? In the Ethernet frame (containing the IP datagram destined to A) that is delivered to router R1, what are the source and destination IP and MAC addresses?
e. Suppose host A would like to send an IP datagram to host B, and neither A’s ARP cache contains B’s MAC address nor does B’s ARP cache A’s MAC address. Further suppose that the switch S1’s forwarding table contains entries for host B and router R1 only. Thus A will broadcast an ARP request message.
i. What actions will switch S1 perform once it receives the ARP request message?
ii. Will router R1 also receive this ARP request message? If so, will R1 forward the message to subnet 3?
iii. Once host B receives this ARP request message, it will send back to host and ARP response message. But will it send an ARP query message to ask for A’s MAC address? Why?
iv. What will switch S1 do once it receives an ARP response message from host B?

Explanation / Answer

a) Data destined to host H1 is forwarded through interface 2.

b). No. Because as per forwarding rule,it is only based on destination address.

c)

VC Router (through B)

Incoming VC Number

Link

Outgoing VC Number

A

20

3

52

B

52

2

98

D

98

3

7

VC Router (through C)

Incoming VC Number

Link

Outgoing VC Number

A

531

4

981

C

981

2

56

D

56

3

12

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