CCNA 3.1.a: Packet handling along the path through a network (FREE LAB!)

Overview:

Understand how a packet is handled by different devices along a path in a network.  In this scenario, the sender is PC1 and the receiver is PC2.

Study Notes:

1. PC1 creates a packet with the destination IP address of PC2
2. PC1 determines that PC2’s IP address is on a different network
3. PC1 looks up its default gateway
4. PC1 looks up the MAC address for its default gateway. If it doesn’t have one, it sends an ARP to get it.
5. PC1 encapsulates the packet in an Ethernet header and sends it to R1
6. R1 receives the frame
7. R1 looks at the destination MAC address, which is the interface on R1
8. R1 copies the frame into its buffer to be processed
9. R1 decapsulates the Ethernet frame and reads the destination IP address
10. R1 determines that the destination IP address is on a different network
11. R1 tries to match the destination IP address and subnet mask with routes in its routing table
12. R1 will select the longest (prefix) match and route the packet accordingly
13. R1 encapsulates the packet for the exit interface and switches the frame to the interface
14. R1 forwards the frame to the next hop.
15. R2 receives the frame
16. R2 looks at the destination MAC address, which is the interface on R2
17. R2 copies the frame into its buffer to be processed
18. R2 decapsulates the Ethernet frame and reads the destination IP address
19. R2 determines that the destination IP address is a directly connected network
20. R2 consults its ARP cache to determine if it has a MAC address for PC2.
21. If it does not, it will send ARPs to get the MAC address for PC2 and add it to the ARP cache
22. R2 encapsulates the packet in a new data-link frame and sends it to PC2
23. The frame arrives at PC2.
24. PC2 examines the destination MAC address and sees that it is its own interface
25. PC2 then copies the rest of the frame, decapsulates it and sends it up the TCP/IP stack for processing by the operating system.

 

THIS LAB FREE!
PacketTracer Lab: CCNA-3.1.a-Packet-handling-through-a-network.pkt

In this lab we will not be making any configuration changes.  Rather we will be initiating a ping and watching as the packet traverses the network.

• In the bottom right corner of PacketTracer, click the stopwatch icon just above where it says Realtime to enter Simulation mode
• Click on PC1
• Select the Desktop tab and then select Command Prompt
• To see your IP settings, type

  ipconfig

• The ping command sends four pings in a single command.  To ping PC2, type

  ping 192.168.0.2

• You will see an envelope appear below PC1
• In the right window click Capture / Forward
• You will see the envelope move across the network to Router1
  • Notice the ARP requests from PC1 to Router1.
• Click Capture / Forward and notice the ARP reply from Router1 back to PC1
• Click Capture / Forward to send the ICMP packet from PC1 to Router1
• Click Capture / Forward to send an ARP to Router2
  • Router2 does not have an ARP entry for 192.168.0.2 so it sends a response to PC1 with Request Timed Out
• Click Capture / Forward to send an ARP request from Router2 to PC2
• Click Capture / Forward to send the ARP response from PC2 to Router2
• Click Capture / Forward to recieve the ICMP response on PC1
  • View the Command Prompt for PC1 and you should see Request Timed Out as the first ping response
• The response to the first ping will be Request Timed Out because Router2 doesn't have an ARP entry for PC2.  Router2 responds to PC1 with Request Timed Out while at the same time completes the ARP with PC2.  The ARP entries now exist in the routers, which is why subsequent pings successfully get a response.

• This is the beginning of second ping.  ARP entries are now populated so you won't see any ARP messages in the next three pings.
• Click Capture / Forward to send the packet from PC1 to Router1
• Click Capture / Forward to send the packet from Router1 to Router2
• Click Capture / Forward to send the packet from Router2 to PC2
• PC2 now takes the ICMP echo request and forms an echo reply message.  To form an echo reply message, the source and destination addresses are simply reversed, the type code changed from 8 to 0 and the checksum is recomputed.
• Click Capture / Forward to send the response packet from PC2 to Router2
• Click Capture / Forward to send the response packet from Router2 to Router1
• Click Capture / Forward to send the response packet from Router1 to PC1
• View the output in the Command Prompt

Reply from 192.168.0.2: bytes=32 time=6ms TTL=126

• Continue clicking Capture / Forward to watch the last 2 pings. They will behave similar to the second ping.