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.

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

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

Overview of 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.

Figure 1-2 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 Figure 1-2, 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 "MPLS Terminology."

As shown in Figure 1-2, the following process takes place in the data forwarding path from R4 to R1:

Architectural Blocks of MPLS

MPLS functionality on Cisco devices is divided into two main architectural blocks:

• Control plane — Performs functions related to identifying reachability to destination prefixes. Therefore, the control plane contains all the Layer 3 routing information, as well as the processes within, to exchange reachability information for a specific Layer 3 prefix. Common examples of control plane functions are routing protocol information exchange like in OSPF and BGP. Hence, IP routing information exchange is a control plane function. In addition, all protocol functions that are responsible for the exchange of labels between neighboring routers function in the control plane as in label distribution protocols (explained in detail in section "LDP Session Establishment").

• Data plane — Performs the functions relating to forwarding data packets. These packets can be either Layer 3 IP packets or labeled IP packets. The information in the data plane, such as label values, are derived from the control plane. Information exchange between neighboring routers creates mappings of IP destination prefixes to labels in the control plane, which is used to forward data plane labeled packets.

Figure 1-3 depicts the control plane and data plane functions.