305344 Principle of Computer Networks 4 ( - PDF document

305344 Principle of Computer Networks บทปฏิบัติการที่ 4 ( อาทิตยที่ 6) ชื่อ _______________________________ รหัส ___________ ขั้นตอนที่ 1 Packet Tracer Lab (ARP) วิธีทํา : ใหศึกษาและทําการทดลองตามขั้นตอน Step 1-Step 4 ใน Packet Tracer Lab: ARP หนาที่ 1-8 ผลการทดลอง ใหเขียนรายงานเพื่อตอบคําถามทายการทดลองขอ Q1 , Q2 , Q3 , และ Q4 ซึ่งมีคะแนน ทั้งหมด 100 คะแนน ( ขอละ 25 คะแนน ) กําหนดสง ใหสงรายงานในชั่วโมงปฏิบัติการของอาทิตยถัดไป (13 ธันวาคม 2549)

Packet Tracer Lab: ARP (100 points) If you need assistance in using Packet Tracer, refer to the lab Using Packet Tracer . Part 1: ARP and the Local Network Step 1: Open the following topology 1

Step 2: Select ARP and ICMP from the Event List Filters Enter Simulation mode and select only the ARP and ICMP protocols selected in the Event List Filters : Step 3: Perform a simple, local Ping Select Add Simple PDU Select Pc0 as the device that sends the ping: Select Pc1 as the device that sends the receives the ping: 2

Q1: (25 points) Step 4: Analyzing the local ping Using the information in the Event List, explain the sequence of events which is described below. Click the forward button and watch as the: 1. Pc0 creates the ICMP ping, Echo Request 2. Pc0 checks its ARP table and does not see an entry for the destination IP Address 3. Pc0 sends an ARP Request (broadcast) which is flooded out by Switch0. 4. Pc1 sends an ARP Reply (unicast) which is filtered by Switch0. 5. Pc0 receives the ARP Reply and adds the IP address and MAC address to its ARP Table (ARP Cache). 6. Pc0 sends the ICMP ping, Echo Request, message that was held in memory. 7. Switch0 filters the ICMP ping, Echo Request. 8. Pc1 sends the ICMP ping, Echo Reply. 9. Switch0 filters the ICMP ping, Echo Reply. 10. Pc0 receives the ICMP ping, Echo Reply. 3

Examine the individual ICMP and ARP packets. Outcome Part 1: Q2 (25 points) • Using this information, explain the information in the following fields for both the ARP Request and the ARP Reply. o Ethernet frame: � Destination MAC Address � Source MAC Address � Type o ARP Message � Opcode � Sender MAC address � Sender IP address � Target MAC address � Target IP address 4

Part 2: ARP and the Remote Network Step 1: Resetting the Network and Waiting for STP Whenever you want to reset the network and begin the simulation again, click on Reset Network in the Event List… Followed by clicking Delete in the PDU area. Waiting for STP: Click on Realtime mode and wait for green lights on all connections. After the green lights are displayed, click on Simulation to return to mode of operation: 5

Step 2: Perform a simple, remote Ping Select Add Simple PDU Select Pc0 as the device that sends the ping and Pc4 as the device to receive the ping: The following steps will occur: 1. Pc0 creates the ICMP ping, Echo Request, with Source IP Address of Pc0 and Destination IP Address of Pc4. 2. Pc0 checks its ARP table and does not see an entry for the IP Address of the default gateway, Router1. 3. Pc0 sends an ARP Request (broadcast) which is flooded out by Switch0. 4. Router1 sends an ARP Reply (unicast) which is filtered by Switch0. 5. Pc0 receives the ARP Reply and adds the IP address and MAC address to its ARP Table (ARP Cache). 6. Pc0 sends the ICMP ping, Echo Request, message that was held in memory. 7. Switch0 filters the ICMP ping, Echo Request. 8. Router1 receives the ICMP ping, and removes the Ethernet frame. 9. The following information is beyond the scope of this lab and will be discussed later. • At this point the router will lookup the Destination IP Address in its routing table. This is beyond the scope of this lab. • Router1 needs to encapsulate the IP packet into an Ethernet frame with the Destination MAC address of Pc4. • Since this information is not in its ARP table, the router issues an ARP Request. • Pc4 returns the ARP Reply. • You will now get the following message from PT: 6

Note : You will notice that the router does an ARP Request for Pc4, but drops the initial, first, ICMP Echo Request. The Cisco IOS is designed so that if the router receives an ICMP ping, Echo Request for a device on an Ethernet interface, it will check its ARP cache (table) to see if the destination IP address and MAC address entry is in its ARP table. If it is it will forward the ICMP ping, Echo Request. If it is not in the ARP table, the router will drop the ICMP ping, Echo Request, but go ahead and issue the ARP Request and wait for the ARP Reply. Since most ping programs use multiple Echo Requests (four or more), the next ICMP ping, Echo Request that comes into the router, the router will have the destination IP address and MAC address entry is in its ARP table from the ARP Reply and forward the ICMP message. This is designed so that the router uses its memory and processing time effectively. Step 3: Analyzing the remote ping Outcome Part 2: Q3 (25 points) • Using this information, explain the information in the following fields for both the ARP Request and the ARP Reply, from Pc0 to Router1. o Ethernet frame: � Destination MAC Address � Source MAC Address � Type o ARP Message � Opcode � Sender MAC address � Sender IP address � Target MAC address � Target IP address Q4 (25 points) Outcome – Part 3 : • How does your computer decide whether the ARP Request is for the Destination IP Address in the packet or the IP Address of the default gateway (assuming there is not an entry in its ARP Table)? 7

Step 4: Perform a second simple, remote Ping DO NOT RESET THE NETWORK Select Add Simple PDU Select Pc0 again and send a second ping to Pc4 : Note : This time you will see that the router forwards the ping to Pc4 because it has its IP address and MAC address in its ARP table from the ARP Request it did in Step 3. 8