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Difficulty: EasyEncapsulation and De-encapsulation

A host computer on Subnet A transmits data to a server located on Subnet B through a Layer 3 router. As the data traverses the router to the destination network, which part of the Protocol Data Unit (PDU) is stripped and replaced with new addressing information at each hop?

  1. The Layer 2 frame header and trailer containing source and destination MAC addressesAnswer
  2. B
    The Layer 3 packet header containing original source and destination IP addresses
  3. C
    The Layer 4 segment header containing source and destination port numbers
  4. D
    The transport protocol specification payload determining TCP or UDP transport selection

Answer

The Layer 2 frame header and trailer containing source and destination MAC addresses
When a router forwards traffic between subnets, it de-encapsulates the incoming Layer 2 frame to extract the Layer 3 IP packet. After consulting its routing table for the next hop, it re-encapsulates the intact Layer 3 packet inside a brand-new Layer 2 frame with new source and destination MAC addresses appropriate for the next physical link.

Step-by-Step Solution

1
Identify the device performing the operation and its operating layer.
The router operates primarily at Layer 3 (Network Layer).
Routers make forwarding decisions based on destination IP addresses.
2
Analyze how encapsulation and de-encapsulation function during hop-by-hop routing.
When a router receives a frame, it de-encapsulates the Layer 2 frame, examines the Layer 3 packet IP header to find the next-hop interface, and re-encapsulates the IP packet into a new Layer 2 frame.
Layer 2 MAC addresses are local to individual broadcast domains and must be recreated for each physical subnet link.

Key Concept

Hop-by-hop Layer 2 re-framing during IP routing
Estimated Time:45s
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Topics

Encapsulation and De-encapsulation

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