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Difficulty: HardEncapsulation and De-encapsulation

A network engineer is tracing traffic from a host on Subnet A to an application server on Subnet B. The traffic traverses a Layer 2 switch, a Layer 3 default gateway router, and a second Layer 2 switch before reaching the destination server. Assuming Network Address Translation (NAT) is not configured, which of the following statements accurately describe how headers are modified or preserved during encapsulation and de-encapsulation across these network devices? (Select TWO.)

  1. The Layer 3 source and destination IP addresses encapsulated by the originating host remain unchanged throughout the path across both subnets.Answer
  2. The default gateway router strips the incoming Layer 2 frame header and encapsulates the IP packet into a new Layer 2 frame before egressing onto the next hop.Answer
  3. C
    The first Layer 2 switch strips the incoming Data Link header and appends a new Layer 3 packet header before forwarding the traffic to the router.
  4. D
    The Layer 4 Transport header is stripped and recreated by the default gateway router at each hop to reset TCP sequence numbers.

Answer

The correct statements are that the Layer 3 source and destination IP addresses remain unchanged across both subnets (when NAT is not used), and the default gateway router strips the incoming Layer 2 frame header before encapsulating the IP packet in a new Layer 2 frame for the destination subnet.
Layer 3 IP headers provide end-to-end addressing and remain intact across routed hops unless NAT is specifically configured. Conversely, Layer 2 frames are link-local; when a router forwards traffic between subnets, it de-encapsulates the incoming frame (stripping the Layer 2 header and trailer) and re-encapsulates the payload packet into a brand new Layer 2 frame appropriate for the next hop.

Step-by-Step Solution

1
Analyze Layer 2 switch operations during transit.
Layer 2 switches evaluate incoming frames at the Data Link layer using MAC address tables and forward the frame intact without stripping or modifying Layer 2, Layer 3, or Layer 4 headers.
Switches do not perform de-encapsulation above Layer 2.
2
Analyze Layer 3 router operations during hop-by-hop forwarding.
The router receives the frame on its ingress interface, performs de-encapsulation by removing the Layer 2 header/trailer, inspects the Layer 3 IP destination address to make a forwarding decision, and then encapsulates the IP packet into a new Layer 2 frame with updated source and destination MAC addresses.
Data Link layer encapsulation is local to each physical network segment/hop.
3
Evaluate end-to-end protocol headers (Layer 3 and Layer 4).
In the absence of NAT or proxying, Layer 3 IP addresses and Layer 4 TCP/UDP headers pass through intermediate routers untouched.
Layer 3 provides logical end-to-end addressing while Layer 4 manages end-to-end communication sessions between hosts.

Key Concept

Hop-by-hop Layer 2 re-framing vs. end-to-end Layer 3/4 header preservation
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Topics

Encapsulation and De-encapsulation

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