This question is based on the two figures shown below. One figure shows the topo

Question

This question is based on the two figures shown below. One figure shows the topology of the network, and the other figure shows the assigned IP address and MAC address. If we replace the router between subnets 1 and 2 with a switch S1, and label the router between subnets 2 and 3 as R1.

a) Consider sending an IP datagram from Host E to Host F. Will Host E ask router R1 to help forward the datagram? In the Ethernet frame containing the IP datagram, what are the source and destination IP and MAC addresses?

b) Suppose E would like to send an IP datagram to B, and assume that E’s ARP cache does not contain B’s MAC address. Will E perform an ARP query to find B’s MAC address? In the Ethernet frame (containing the IP datagram destined to B) that is delivered to router R1, what are the source and destination IP and MAC addresses?

c) 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 contain 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. What actions will switch S1 perform once it receives the ARP request message? Will router R1 also receive this ARP request message? If so, will R1 forward the message to Subnet 3? Once Host B receives this ARP request message, it will send back to Host A an ARP response message. But will it send an ARP query message to ask for A’s MAC address? Why? What will switch S1 do once it receives an ARP response message from Host B?

d) Suppose now that the router between subnets 2 and 3 is replaced by a switch. Answer questions (a)(b) in the previous problem in this new context.

Explanation / Answer

a). No Host E doe not ask Router 1 to forward as it checks for Host F ip address it is in the same LAN it will send to router 2 and will not( no need) to send to router 1

ethernet frame is

Souce IPaddress:192.169.3.001

Destination IPaddress:192.168.3.003

Source MAC :77-77-77-77-77-77

Destination MAC:99-99-99-99-99-99

b).No E will not perform query because it checks that Host B IP address is not in the same LAN

ethernet frame is

Souce IPaddress:192.169.3.001

Destination IPaddress:192.168.1.003

Source MAC :77-77-77-77-77-77

Destination MAC:33-33-33-33-33-33 (since via router1 )

c)As the received ARP frame's destination address is a broadcast address S1 broadcast the ethernet frame and finds Host A belongs to LAN which is connected to Subnet1 and it updates the table with Host A's entry.Router 1 also receives this message as it is a broadcasst message but will not forward to subnet 3 which is of different subnet but B receive this ARP message so it does not ask for ARP query to find Host A mac address as it already knew it from ARP response.when S1 receives B's response message it also adds Host B's entry and sends packet as both are in same subnet connected through S1

d)The answer remains same even if the router is interchanged with switch for a and b bits

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