Unicast IP Forwarding in Traditional IP Networks
In traditional IP networks, routing protocols are used to distribute Layer 3 routing information. 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
| 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 Forwarding
In MPLS enabled networks, packets are forwarded based on labels. These labels might correspond to IP destination addresses or to other parameters, such as QoS classes and source address. Labels are generated per router (and in some cases, per interface on a router) and bear local significance to the router generating them. Routers assign labels to define paths called Label Switched Paths (LSP) between endpoints. Because of this, only the routers on the edge of the MPLS network perform a routing lookup.
illustrates the same network as depicted in Figure 1-1 with MPLS forwarding where route table lookups are performed only by MPLS edge border routers, R1 and R4. The routers in MPLS network R1, R2, and R3 propagate updates for 172.16.10.0/24 network via an IGP routing protocol just like in traditional IP networks, assuming no filters or summarizations are not configured. This leads to the creation of an IP forwarding table. Also, because the links connecting the routers are MPLS enabled, they assign local labels for destination 172.16.10.0 and propagate them upstream to their directly connected peers using a label distribution protocol; for example, R1 assigns a local label L1 and propagates it to the upstream neighbor R2. R2 and R3 similarly assign labels and propagate the same to upstream neighbors R3 and R4, respectively. Consequently, as illustrated in, the routers now maintain a label forwarding table to enable labeled packet forwarding in addition to the IP routing table. The concept of upstream and downstream is explained in greater detail in the section.
