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The figure shows the building blocks used by routers to perform traditional IP forwarding.

The control plane consists of a routing protocol that exchanges routing information and maintains the contents of the main routing table. When combined with Cisco Express Forwarding (CEF), the IP forwarding table in the data plane forwards the packets through the router.

The LDP (Label Distribution Protocol) (or the Cisco proprietary protocol TDP, or Tag Distribution Protocol) in the control plane exchanges labels and stores them in the label information base (LIB). This information is then used in the data plane to provide MPLS functionality:

■ A label is added to the IP forwarding table (Forwarding Information Base, or FIB) to map an IP prefix to a next-hop label.

■ A locally generated label is added to the label forwarding information base (LFIB) and mapped to a next-hop label.

The following forwarding scenarios are possible when MPLS is enabled on a router:

■ An incoming IP packet is forwarded by using the FIB table and sent out as an IP packet (the usual CEF switching).

■ An incoming IP packet is forwarded by using the FIB table and sent out as a labeled IP packet (the default action if there is a label assigned to the destination IP network).

■ An incoming labeled packet is forwarded by using the LFIB table and sent out as a labeled IP packet.

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