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Difficulty: MediumSeparation of Control Plane and Data Plane

A network administrator notices that while transit data packets are forwarded rapidly across a router using hardware ASICs, traceroute traffic whose IP TTL expires at the router causes a temporary increase in main CPU utilization. Which architectural plane is directly responsible for processing these TTL-expired packets and generating the resulting ICMP Time Exceeded messages?

  1. A
    Data plane
  2. Control planeAnswer
  3. C
    Management plane
  4. D
    Policy plane

Answer

The control plane is directly responsible for processing exception packets, such as generating ICMP Time Exceeded messages when an IP packet's Time-To-Live (TTL) reaches zero.
The control plane is managed by the network device's CPU and is responsible for running routing protocols (like OSPF and BGP) as well as handling exception packets. When a transit IP packet's TTL expires, the data plane cannot complete hardware forwarding and punts the packet to the control plane CPU to construct and send an ICMP Time Exceeded packet.

Step-by-Step Solution

1
Analyze the packet handling requirement in the scenario.
The packet has an expired TTL (TTL=1 upon arrival at the Layer 3 device), requiring an ICMP Time Exceeded response to be crafted.
Hardware ASICs in the data plane are optimized for standard forwarding lookups using the FIB and TCAM, but cannot originate complex control messages.
2
Identify which plane receives exception or punted traffic.
The data plane punts the packet to the CPU, which operates in the control plane.
Control processor software handles routing protocol updates, ARP request/reply generation, and ICMP exception message creation.

Key Concept

Control Plane vs Data Plane Exception Handling
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Topics

Separation of Control Plane and Data Plane

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