Routing Decisions

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.

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.