Unicast IP Forwarding in Traditional IP Networks

In traditional IP networks, routing protocols are used to distribute Layer 3 routing information. Figure 1-1 depicts a traditional IP network where network layer reachability information (NLRI) for network 172.16.10.0/24 is propagated using an IP routing protocol. Regardless of the routing protocol, packet forwarding is based on the destination address alone. Therefore, when a packet is received by the router, it determines the next-hop address using the packet's destination IP address along with the information from its own forwarding/routing table. This process of determining the next hop is repeated at each hop (router) from the source to the destination except in the case of policy-based routing where a certain outbound policy might affect packet forwarding.

Figure 1-1. Traditional IP Forwarding Operation

As shown in Figure 1-1, in the data forwarding path, the following process takes place:

1.	R4 receives a data packet destined for 172.16.10.0 network.
2.	R4 performs route lookup for 172.16.10.0 network in the forwarding table, and the packet is forwarded to the next-hop Router R3.
3.	R3 receives the data packet with destination 172.16.10.0, performs a route lookup for 172.16.10.0 network, and forwards the packet to next-hop Router R2.
4.	R2 receives the data packet with destination 172.16.10.0, performs a route lookup for 172.16.10.0 network, and forwards the packet to next-hop Router R1.

Because R1 is directly connected to network 172.16.10.0, the router forwards the packet on to the appropriate connected interface.