Free CCNA | Life of a Packet | Day 12 Lab | CCNA 200-301 Complete Course

Welcome to Jeremy’s IT Lab

Introduction to the CCNA Course

• Jeremy introduces a free, complete course for the CCNA and encourages viewers to subscribe, like, comment, and share the video to help spread awareness of the series.

• Viewers are reminded to download a packet tracer practice lab linked in the description for hands-on learning.

Understanding Packet Travel

Key Questions on MAC Addresses

• The session focuses on three main questions regarding source and destination MAC addresses as packets travel through a network. The first scenario involves PC1 pinging PC4 across different networks (192.168.1.0/24 and 192.168.3.0/24).

• It is emphasized that while IP addresses remain constant during transmission, MAC addresses will vary based on device interfaces involved in routing the packet from source to destination.

Analyzing Source/Destination MAC Addresses

• For part A of question 1, when PC1 sends a packet to SW1, its source MAC address is PC1's MAC address while the destination is R1's G0/0 interface's MAC address (the default gateway).

• In part B, between SW1 and R1, the source and destination MAC addresses remain unchanged as SW1 forwards frames without altering them; it learns about PC1’s MAC if not already known.

Practical Demonstration of ARP Process

Sending Pings for Verification

• To verify answers regarding MAC addresses, Jeremy initiates a ping from PC1 to PC4 which allows ARP processes to complete before analyzing results in simulation mode. Initial pings may time out due to ongoing ARP resolution processes but subsequent pings succeed once devices learn each other's addresses.

Checking Device MAC Addresses

• Commands are demonstrated for checking physical (MAC) addresses:

• On Windows: IPCONFIG /ALL reveals PC1's last four digits of its physical address as 1111.

• On R1: Using SHOW INTERFACE G0/0, it shows R1's last four digits as AAAA with an explanation about BIA (Burned In Address) being another term for a device’s original configured MAC address by Cisco during manufacturing.

Simulation Mode Analysis

Confirming Frame Details

• In simulation mode after sending another ping from PC1:

• The ICMP packet details show that at layer 2 (Ethernet header), data travels from PC1 (1111) to R1’s G0/0 interface (AAAA), confirming previous analysis.

Ethernet Packet Transmission and MAC Addressing

Understanding Source and Destination MAC Addresses

• The packet transmitted from R1 uses the MAC address of its G0/1 interface (last 4 digits: BBBB) as the source, while R2’s G0/0 interface (last 4 digits: CCCC) serves as the destination.

• Upon checking the packet at R1, it confirms that the source MAC is BBBB and the destination MAC is CCCC before forwarding to R2.

Forwarding Between Routers

• At R2, the next hop for forwarding involves using its G0/1 interface's MAC address (last 4 digits: DDDD) as the source and R3’s G0/0 interface (last 4 digits: EEEE) as the destination. This is verified through CLI commands.

• The packet is then sent to R3 with confirmed source DDDD and destination EEEE addresses, matching predictions made earlier in the process.

Direct Communication to End Devices

• For communication between R3 and SW2, since PC4 belongs to a directly connected network (192.168.3.0/24), R3 sets PC4’s MAC address as the destination while using its own address (FFFF) as the source for both parts E and F of this question.

• Verification shows that both IN layers at SW2 reflect a source MAC ending with FFFF and a destination ending with 4444, confirming accuracy in addressing throughout transmission steps.

Ping Operation Between PCs

Differences in Packet Handling

• In question 2, when PC1 pings PC3 on the same network, it does not send packets to a default gateway but directly communicates with PC3 instead; SW1 merely forwards frames without altering any addresses.

• Both A and B require identifying PC1’s MAC address (1111) as source and PC3’s MAC address (3333) as destination after confirming via IPCONFIG command on PC3's CLI.

Simulation of Ping Process

• After initiating a ping from PC1 to PC3, verification shows that upon entering SW1 via F0/1 and exiting via F0/3, both source (1111) and destination (3333) MAC addresses remain unchanged during transit through SW1.

Conclusion & Engagement Request

Invitation for Interaction

• Viewers are encouraged to post their answers regarding question 3 in comments; questions will be pinned below for reference but will follow similar formats used previously without requiring actual MAC addresses inputted by users.