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Difficulty: EasySeparation of Control Plane and Data Plane

An engineer is inspecting traffic processing on a core router during a network maintenance window. Which operation is handled directly within the data plane (forwarding plane) of the router?

  1. Decrementing the IP Time-to-Live (TTL) field and rewriting Layer 2 headersAnswer
  2. B
    Building and updating the Routing Information Base (RIB) from dynamic protocols
  3. C
    Handling interactive SSH sessions and processing network management commands
  4. D
    Establishing BGP neighbor relationships and exchanging topology updates

Answer

Decrementing the IP Time-to-Live (TTL) field and rewriting Layer 2 headers
The data plane (forwarding plane) deals exclusively with forwarding user transit traffic across the network device. Operations performed on a per-packet basis—such as decrementing the IP TTL value, updating packet checksums, performing FIB lookups, and applying Layer 2 framing encapsulation—are executed directly in data plane hardware (ASICs).

Step-by-Step Solution

1
Identify the primary role of the data plane (forwarding plane).
The data plane is responsible for processing transit packets through fast-path hardware (ASICs and TCAM) using pre-populated forwarding tables.
Per-packet actions must take place at wire speed without involving the main system CPU.
2
Evaluate which listed operation occurs on every passing frame in hardware.
Modifying packet headers (such as decrementing the TTL), checking/recalculating IP checksums, looking up destination addresses in the Forwarding Information Base (FIB), and rewriting Layer 2 headers are core data plane tasks.
Routing protocol computations (RIB creation) and SSH management belong to the control plane and management plane respectively.

Key Concept

Data plane hardware forwarding operations vs. control plane routing intelligence
Estimated Time:45s
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Topics

Separation of Control Plane and Data Plane

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