Router Switching Function

In this article we will discuss Router Switching Function, will make brief discussion on Router Switching Function, In last article we discuss about Command History Feature.

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 2 frame header and trailer to expose the Layer 3 packet.

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.

Devices have Layer 3 IPv4 addresses and Ethernet interfaces have Layer 2 data link addresses. For example, PC1 is configured with IPv4 address 192.168.1.10 and an example MAC address of 0A-10. As a packet travels from the source device to the final destination device, the Layer 3 IP addresses do not change. This is because the Layer 3 PDU does not change. However, the Layer 2 data link addresses change at every hop as the packet is de-encapsulated and re-encapsulated in a new Layer 2 frame by each router.

It is common for packets to require encapsulation into a different type of Layer 2 frame than the one which was received. For example, a router might receive an Ethernet encapsulated frame on a FastEthernet interface, and then process that frame to be forwarded out of a serial interface.

Notice in the figure that the ports between R2 and R3 do not have associated MAC addresses. This is because this is a serial link. MAC addresses are only required on multi-access networks, such as Ethernet. A serial link is a point-to-point connection and uses a different Layer 2 frame that does not require the use of a MAC address. In this example, when Ethernet frames are received on R2 from the Fa0/0 interface, destined for PC2, it is de-encapsulated and then re-encapsulated for the serial interface, such as a Point-to-Point Protocol (PPP) encapsulated frame. When R3 receives the PPP frame, it is de-encapsulated again and then re-encapsulated into an Ethernet frame with a destination MAC address of 0B-20, prior to being forwarded out the Fa0/0 interface.

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