2. KR, Ch. 6- P15 (slightly reworded with a modified figure). Consider the figur

Question

2. KR, Ch. 6- P15 (slightly reworded with a modified figure). Consider the figure below where S1 and S2 are switches and RI is a router. Hosts A, B, C, and D belong to Subnet 1/2 (in this modification of Figure 6.33 of the text), while hosts E and F belong to Subnet 3 (a) Consider sending an IP datagram from Host E to Host F. Will Host E ask router RI to help forward the datagram? Why? 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? Why? In the Ethernet frame (containing the IP datagram destined to B) that is delivered to router Ri, 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 the 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 sent 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? host S1 R1 S2 host host host subnet 1/2 subnet 3

Explanation / Answer

a
If the Host E wants to send a datagram to Host F, and if Host E is aware of Host F's MAC address it will send a frame similar to:
Source IP: Host E IP
Source MAC: Host E MAC
Destination IP: Host F IP
Destination MAC: Host F MAC

Switch S2 reading this frame will forward the frame to Host E and no Router interfarence is required

b
E only needs to know the IP address of the of Host B. Host E should send a frame with following headers:
Source IP: Host E IP
Source MAC: Host E MAC
Destination IP: Host B IP
Destination MAC: Router R1's Interface 2 MAC

This frame will reach the Router R1 and router will forward the packet to Switch S1 with the following header:
Source IP: Host E IP
Source MAC: Router R1's Interface 1 MAC
Destination IP: Host B IP
Destination MAC: Host B MAC

c
Switches are not smart enough to handle ARP. Host A will broadcast a ARP request. The packet will be received all devices connected to the Switch S1. So both Host B and Router R1 will receive the broadcast message. Router R1 will not forward the message, since ARP request does not leave the subnet network. On other hand Host B replies back which contains Host B's MAC address. But Host B will request for A's MAC address, since the received packet already contains the Host A's MAC address. So as Host B replies the ARP request, the Switch S1 will receive the packet, and will open a Point to point connection with A and packet will be transfered to A. Switch S1 will not broadcast the message because when Host A requested for ARP, Switch S1 has registered Host A's MAC and Interface.

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