Routing Decisions for Cisco Exam 200-125: Cisco Certified Network Associate Exam

Routing Decisions

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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?



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A. B. C. D.


Router Switching Function ( 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.


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.

When a router receives a packet from one network and needs to forward it to another network, it needs to make a routing decision to determine the best path to the destination network. During this process, the router examines the Layer 3 IP address in the packet's header to determine the destination network.

Once the router determines the best path to the destination network, it replaces the Layer 2 frame header of the packet with a new header that contains the MAC address of the next hop router or the destination host on the same network. The Layer 2 frame trailer is not modified.

Therefore, the correct answer to the question is A. Layer 2 frame header and trailer. The Layer 3 IP address, Layer 5 session, and Layer 4 protocol are not replaced by the router during the routing process. The Layer 3 IP address is used by the router to make a routing decision, while the Layer 4 protocol and Layer 5 session information are used by the receiving and sending hosts to establish and maintain communication.