EP

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.