Path Determination

Packet forwarding involves two functions:
Path determination function
Switching function
The path determination function is the process of how the router determines which path to use when forwarding a packet. To determine the best path, the router searches its routing table for a network address that matches the packet's destination IP address.
One of three path determinations results from this search:
Directly Connected Network - If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the router's interfaces, that packet is forwarded directly to that device. This means that the destination IP address of the packet is a host address on the same network as this router's interface.
Remote Network - If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.
No Route Determined - If the destination IP address of the packet does not belong to either a connected or remote network, and if the router does not have a default route, then the packet is discarded. The router sends an ICMP unreachable message to the source IP address of the packet.
In the first two results, the router re-encapsulates the IP packet into the Layer 2 data link frame format of the exit interface. The type of Layer 2 encapsulation is determined by the type of interface. For example, if the exit interface is FastEthernet, the packet is encapsulated in an Ethernet frame. If the exit interface is a serial interface configured for PPP, the IP packet is encapsulated in a PPP frame.

The following section demonstrates this process.


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