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Difficulty: MediumEncapsulation and De-encapsulation

A host workstation on Subnet A transmits data to a server located on Subnet B across an intermediate IP router. Which of the following best describes the header modifications that occur when the router processes and forwards the data unit to the next hop?

  1. The Layer 2 source and destination MAC addresses are updated for the next hop, while the Layer 3 source and destination IP addresses remain unchanged.Answer
  2. B
    The Layer 3 source and destination IP addresses are updated for the next hop, while the Layer 2 source and destination MAC addresses remain unchanged.
  3. C
    Both the Layer 2 MAC addresses and Layer 3 IP addresses are modified at each intermediate routing hop.
  4. D
    The Layer 4 source and destination port numbers are modified for the next hop, while the Layer 2 frame header remains unchanged.

Answer

The Layer 2 source and destination MAC addresses are updated for the next hop, while the Layer 3 source and destination IP addresses remain unchanged.
During routing, the router de-encapsulates the incoming Layer 2 frame, inspects the Layer 3 IP packet header to determine the outgoing interface and next hop, and then re-encapsulates the IP packet into a new Layer 2 frame. The new frame uses the router egress interface MAC address as the source MAC and the next-hop device MAC address as the destination MAC. The IP header source and destination addresses remain unchanged.

Step-by-Step Solution

1
Analyze the role of Layer 2 framing across network boundaries.
Layer 2 headers (MAC addresses) are hop-by-hop parameters that only deliver frames between directly connected interfaces on the same physical link.
When a router receives a frame, it strips the incoming Layer 2 header during de-encapsulation.
2
Analyze the role of Layer 3 IP headers across network boundaries.
Layer 3 headers (IP addresses) contain the original source host IP and final destination host IP.
IP headers provide end-to-end logical addressing and remain unchanged across intermediate standard routers.
3
Determine the re-encapsulation process executed by the forwarding router.
The router encapsulates the IP packet into a new Layer 2 frame containing its egress interface MAC address as the source and the target next-hop interface MAC address as the destination.
This enables physical delivery across the next network segment.

Key Concept

Hop-by-hop Layer 2 re-framing vs. end-to-end Layer 3 packet encapsulation
Estimated Time:1m 30s
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Topics

Encapsulation and De-encapsulation

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