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Difficulty: MediumEncapsulation and De-encapsulation

A client workstation on a corporate LAN transmits web traffic to an external server across multiple subnets. When an intermediate router receives the Ethernet frame on its ingress interface, which of the following best describes the encapsulation and de-encapsulation actions performed by the router prior to forwarding the packet out its egress interface?

  1. The router strips the incoming Layer 2 frame header and trailer to inspect the Layer 3 packet, then encapsulates the original Layer 3 packet into a new Layer 2 frame formatted for the egress link.Answer
  2. B
    The router strips both the Layer 2 frame encapsulation and the Layer 3 IP header, encapsulating the raw Layer 4 TCP segment directly into a new Layer 2 frame for the next hop.
  3. C
    The router de-encapsulates the frame up to Layer 4 to modify source and destination port numbers before rebuilding new Layer 3 and Layer 2 headers.
  4. D
    The router forwards the entire Layer 2 frame without removing its header or trailer, preserving the original source and destination MAC addresses throughout the path.

Answer

The router removes the incoming Layer 2 frame header and trailer, inspects the Layer 3 packet header to make a forwarding decision, decrements the Time to Live (TTL), and encapsulates the untouched IP packet inside a new Layer 2 frame for the outbound interface.
When traffic crosses a router (Layer 3 device), the router strips the incoming Layer 2 frame header and trailer because MAC addresses are only locally significant. The router inspects the Layer 3 IP header to determine the next hop, decrements the TTL, and then wraps the original IP packet inside a new Layer 2 header and trailer formatted specifically for the next hop's data link protocol.

Step-by-Step Solution

1
De-encapsulate Layer 2
The router receives the physical signals on its ingress interface, validates the Frame Check Sequence (FCS), and strips the Layer 2 Ethernet header and trailer.
Layer 2 encapsulation is only valid for local communication within the ingress network segment.
2
Inspect Layer 3 IP Header
The router reads the destination IP address in the Layer 3 packet header, consults its routing table, decrements the TTL field, and re-calculates the IP header checksum.
Layer 3 payload (the IP packet, including Layer 4 TCP and upper-layer data) remains intact to maintain end-to-end transport integrity.
3
Encapsulate for Egress Layer 2
The router adds a new Layer 2 header (with the egress interface MAC as source and next-hop MAC as destination) and a new trailer around the IP packet.
New Layer 2 encapsulation is required to transmit the packet across the next physical hop toward the destination.

Key Concept

Hop-by-hop Layer 2 de-encapsulation and re-encapsulation by Layer 3 devices
Estimated Time:1m 30s
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Topics

Encapsulation and De-encapsulation

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