CCNA | Life of a Packet | 12

Introduction to Packet Sending

• Overview of the video’s focus: Sending a packet to a remote destination.

• Key topics include ARP, encapsulation, and de-encapsulation.

• Tailored for students preparing for the CCNA exam, avoiding complexities suited for CCNP or CCIE.

Network Topology

• Packet journey from PC 1 (192.168.1.24) to PC 4 (192.168.4.24).

• Preconfigured static routing for consistency with previous videos.

• Devices involved:

  • PC 1 to R1, R2, R4, and then PC 4 (specific routing path acknowledged).

MAC Address Assignments

• Assigning MAC addresses for clarity:

  • PC 1: 1111

  • R1: G0/2 - AAAA, G0/0 - BBBB

  • R2: G0/0 - CCCC, G0/1 - DDDD

  • R4: G0/1 - EEEEE, G0/2 - FFFE

  • PC 4: 4444

• Note on unique MAC addresses per interface.

• Switch MAC addresses not detailed, to avoid clutter.

Sending the Packet - Initial Steps

• PC 1 encapsulates data with an IP header:

  • Source IP: 192.168.1.1

  • Destination IP: 192.168.4.1

• Recognizes the need to contact its default gateway (R1) due to different networks.

ARP Process

• PC 1 generates an ARP request to discover R1's MAC address:

  • Source IP: PC 1's IP 192.168.1.1

  • Destination IP: R1’s G0/2 (192.168.1.254).

  • Destination MAC: Broadcast (all F's, indicating unknown)

  • Source MAC: PC 1’s MAC address (1111).

• Switch 1 forwards the frame to R1 after broadcasting to all connected devices.

R1 Responds with ARP

• R1 receives the ARP request, recognizes its own IP address.

• Crafts an ARP reply to PC 1:

  • Source: R1's MAC (AAAA)

  • Destination: PC 1's MAC (1111).

• Frame forwarding understanding - Switches learn MAC addresses based on incoming frames.

Encapsulating the Packet for R2

• After the ARP process, R1 encapsulates the original packet for delivery:

  • Sets R2's MAC address as the destination for the Ethernet frame.

• Recognizes need for R2’s MAC due to no previous interaction.

Using ARP Again - R1 to R2

• R1 creates an ARP request:

  • Source IP: R1’s IP

  • Destination: R2’s IP (192.168.12.2), broadcast MAC address.

• R2 replies to R1's request with its MAC address.

R2 to R4 Transition

• R2 now encapsulates the packet for delivery to R4:

  • Uses ARP for R4's MAC address due to lack of direct knowledge.

  • R2 forwards an ARP request to R4.

• R4 replies with its own MAC address.

Final Delivery to PC 4

• R2 encapsulates the packet for R4:

  • Uses R4's MAC address as destination.

• R4 sends out an ARP request to learn PC 4's MAC address.

• PC 4 responds, completing the communication cycle.

Key Observations

• Throughout the process, the original packet remains unchanged:

  • Source IP: 192.168.1.1, Destination IP: 192.168.4.1.

• Switches do not modify frames; they only learn MAC addresses and forward.

Replies and Subsequent Communication

• In a return packet from PC 4 to PC 1, no ARP requests needed due to prior ARP resolution.

• Packets pass unchanged back through the routing devices via de-encapsulation and re-encapsulation.

Quiz Preview

• Approach to testing understanding: Illustrated diagram rather than multiple choice.

• Participants will engage with specific questions based on the packet sending process discussed.