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

Question

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

Now replace router between subnets 1 and 2 with a switch in Figure 2 below. Label that switch S1 and the router between subnets 2 and 3 R1.

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

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?

Ans: No, the router will not have to. why because, F will check the subnet prefix of Host E’s IP address. And then we know that E is in the same LAN. So, F won't send the packet to R1.The switch in the subnet will have a forwarding table that contains where to forward the message, so therefore the switch will have no reason to forward the message to the router. The source IP and MAC will be that of the switch that is forwarding the message. The destination will be that of Host E.

Source IP : F’s IP address

Destination IP : E’s IP address

Source MAC : F’s MAC address

Destination MAC : E’s MAC address

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?

Ans : F will perform an ARP query to find A’s MAC address. This is because it does not know where Host A is, Because F can know that they are not on the same LAN by checking A’s IP address. So it much try to locate and get the IP and MAC that it wants to send to. The source will be the IP and MAC of the router, because A needs to return the message to the router in order to reach F. The destination will the that of Host A.

Source IP : F’s IP address

Destination IP : A’s IP address

Source MAC : F’s MAC address

Destination MAC : The MAC address of R1’s interface connecting to Subnet 3

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.

Ans :S1 will forward the ARP request out to Host B since S1 already contains B’s information. This means that R1 will not receive the request since S1 will have no reason to send it to anyone other that Host B. Host B will have to send out an ARP request because it does not have Host A’s MAC address. S1 will log the MAC and IP of Host A when it receives the response, then it will forward the message onto Host B.

i. What actions will switch S1 perform once it receives the ARP request message?

Switch S1 will broadcast the Ethernet frame via both its interfaces when the received ARP frame. Because its destination address is a broadcast address. And it learns that A is on Subnet 1 which is connected to S1 at a certain interface. Then S1 will update its forwarding table to include Host A.

ii. Will router R1 also receive this ARP request message? If so, will R1 forward the message to subnet 3?

R1 also receives this ARP request message, but it won’t 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?

B won’t send ARP query message to ask for A’s MAC address. Because B can know it from A’s query message (A and B are on the same subnet1).

iv. What will switch S1 do once it receives an ARP response message from host B?

When switch S1 receives B’s response message, it will add an entry for host B in its forwarding table, and then drop the received frame as destination host A is on the same interface as host B.

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