7.5.1 Packet Tracer – Investigate the Layer 2 Frame Headers Answers
Topology
Learning Objectives
- Examine the network
- Run the simulation
Introduction:
As an IP packet crosses an internetwork it can be encapsulated in many different Layer 2 frames. Packet Tracer supports Ethernet, Cisco’s proprietary HDLC, IETF standards based PPP, and Frame Relay at Layer 2. As the packet is passed from router to router, the Layer 2 frame is stripped off, and the packet is encapsulated in the Layer 2 frame for the outbound interface. This activity will follow an IP packet across an internetwork and examine the different Layer 2 encapsulations.
Task 1: Examine the network
Step 1. Examine the links between routers
PC1 connects to PC2 through four routers. The three links between these routers each use a different Layer 2 encapsulation. The link between Cisco1 and Cisco2 uses Cisco’s proprietary HDLC. The link between Cisco2 and Brand X uses IETF standards based PPP since Brand X is not a Cisco router. The link between Brand X and Cisco3 uses Frame Relay through a service provider network to reduce the cost compared with that of a dedicated link.
Step 2. Verify connectivity in Realtime mode
Ping the IP address of PC2 from the Command Prompt of PC1. Use the command ping 192.168.5.2. If the ping times out, repeat the command until it succeeds. It may take several tries until the network converges.
Task 2: Run the simulation
Step 1. Start the simulation
Enter Simulation mode. The PDU at PC1 is an ICMP echo request intended for PC2. Click the Capture / Forward button twice until the PDU reaches router Cisco1.
Step 2. Examine the Layer 2 encapsulation
Click on the PDU at router Cisco1. The PDU Information window opens. Click the Inbound PDU Details tab. The inbound Layer 2 encapsulation is Ethernet II as the frame arrives from the LAN. Click the Outbound PDU Details tab. The outbound Layer 2 encapsulation is HDLC as the frame is about to be sent to router Cisco2.
Click the Capture / Forward button again. Repeat the process as the PDU arrives at each router along the path to PC2. Notice the change in the Layer 2 encapsulation at each hop. Notice, also, that the encapsulated IP packet does not change.
