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Difficulty: MediumEncapsulation and De-encapsulation

A network administrator is analyzing traffic flow across a Layer 3 router connecting two separate physical IP subnets. As an IP packet travels from Host A on Subnet 1 through the router to Host B on Subnet 2, which of the following best describes how PDU encapsulation headers are processed and modified by the router?

  1. The router strips the incoming Layer 2 Ethernet header and trailer, decrements the TTL in the Layer 3 IP header, and encapsulates the packet into a new Layer 2 frame with updated MAC addresses for the destination subnet.Answer
  2. B
    The router strips both the Layer 2 Ethernet header and the Layer 3 IP header, re-encapsulating the underlying Layer 4 segment with new IP addresses and MAC addresses suitable for the egress link.
  3. C
    The router preserves the incoming Layer 2 frame header intact and only modifies the destination IP address within the Layer 3 header to match the router's egress interface.
  4. D
    The router de-encapsulates the frame completely up to Layer 7 to inspect payload data before generating a new transport segment, network packet, and data link frame.

Answer

The router strips the incoming Layer 2 Ethernet header and trailer, decrements the TTL in the Layer 3 IP header, and encapsulates the packet into a new Layer 2 frame with updated MAC addresses for the destination subnet.
When a packet traverses a Layer 3 router, the router de-encapsulates the incoming Layer 2 frame to inspect the Layer 3 destination IP address. The router updates the IP header TTL field, determines the next hop via its routing table, and encapsulates the unmodified IP packet inside a new Layer 2 Ethernet frame formatted with source and destination MAC addresses specific to the egress link.

Step-by-Step Solution

1
Analyze incoming frame processing at the router ingress interface
The router receives the Ethernet frame, verifies the Frame Check Sequence (FCS), and de-encapsulates the Layer 2 header and trailer.
Layer 2 headers are valid only for a single network hop/segment and must be stripped to access the Layer 3 packet.
2
Inspect Layer 3 IP header and execute routing decision
The router reads the destination IP address, decrements the Time to Live (TTL) count by 1, and recalculates the IP header checksum.
The IP header remains intact to maintain end-to-end logical addressing across the path, except for hop-limiting fields.
3
Re-encapsulate the packet for the egress network segment
The IP packet is encapsulated into a new Layer 2 frame formatted for the destination link, using the router egress interface MAC as source and the next hop/host MAC as destination.
Encapsulation creates a valid data link layer frame for delivery across the next physical or logical link.

Key Concept

Hop-by-hop Layer 2 de-encapsulation and re-encapsulation by Layer 3 routing devices
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Topics

Encapsulation and De-encapsulation

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