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Difficulty: HardEncapsulation and De-encapsulation

A network administrator analyzes traffic captures taken at different network segments along a multi-hop path while Host A transmits a file to Host B across a standard Layer 3 router (without NAT). Which of the following statements correctly describe how protocol headers and Data Link trailers are altered or processed during this network traversal? (Select TWO.)

  1. The Layer 2 Data Link frame header and trailer are stripped at the router's ingress interface and encapsulated with a new source MAC and next-hop destination MAC at the egress interface.Answer
  2. The Layer 3 IP header retains the original source and destination IP addresses from Host A to Host B throughout the router traversal.Answer
  3. C
    The Layer 3 IPv4 packet header is completely removed by the router and replaced with a new network header specifying the router egress interface IP address as the source.
  4. D
    The Layer 4 TCP segment header is re-encapsulated by intermediate Layer 2 switches to assign temporary port numbers for each switchport traversal.

Answer

The correct statements are that Layer 2 frame headers and trailers are stripped and replaced at each router hop, and that Layer 3 IP headers preserve the original end-to-end IP addresses across standard routing hops.
Layer 2 framing (MAC headers and FCS trailers) operates strictly on a hop-by-hop basis, meaning a router must strip the incoming link-layer frame and construct a brand-new frame for the next segment. Simultaneously, Layer 3 logical addressing (source and destination IP addresses) remains unchanged end-to-end across standard routing hops.

Step-by-Step Solution

1
Analyze Layer 2 scope during router traversal.
Layer 2 addressing (MAC addresses, Ethernet headers, FCS trailers) is local to a single collision/broadcast domain. Routers terminate Layer 2 broadcast domains, stripping inbound frames and encapsulating outbound IP packets into new frames suited for the next-hop link.
Data Link encapsulation is hop-by-hop.
2
Analyze Layer 3 scope during router traversal.
Layer 3 IP headers contain end-to-end addressing. Routers update the Time-to-Live (TTL) field and recalculate the IP checksum, but the original source IP and final destination IP remain constant across intermediate standard routers.
Network layer addressing provides end-to-end logical path identification.
3
Evaluate intermediate switch and router processing against OSI layer roles.
Intermediate Layer 2 switches do not alter Layer 4 TCP port numbers or headers, nor do routers rebuild Layer 3 headers from scratch.
Encapsulation layers remain isolated; devices only modify fields relevant to their operational layer and lower-layer framing.

Key Concept

Hop-by-Hop Data Link Encapsulation vs. End-to-End Network Address Preservation
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Topics

Encapsulation and De-encapsulation

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