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.





MPLS Configuration on Cisco IOS Software
MPLS Configuration on Cisco IOS Software
ISBN: 1587051990
EAN: 2147483647
Year: 2006
Pages: 130
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