Cisco 200-125 Exam

When a router makes a routing decision for a packet that is received from one network and destined to another, which portion of the packet does if replace?

  • A. Layer 2 frame header and trailer
  • B. Layer 3 IP address
  • C. Layer 5 session
  • D. Layer 4 protocol
Answer: Option A. Explanation
Explanation: 

Router Switching Function (1.2.1.1) A primary function of a router is to forward packets toward their destination. This is accomplished by using a switching function, which is the process used by a router to accept a packet on one interface and forward it out of another interface. A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link.
NOTE:
In this context, the term “switching” literally means moving packets from source to destination and should not be confused with the function of a Layer 2 switch.
After the router has determined the exit interface using the path determination function, the router must encapsulate the packet into the data link frame of the outgoing interface. What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps:
Step 1. De-encapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer.
Step 2. Examines the destination IP address of the IP packet to find the best path in the routing table.
Step 3. If the router finds a path to the destination, it encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.

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