LDP performs dynamic label distribution in MPLS environments. The LDP operation begins with a Hello discovery process network to form an adjacency with an LDP peer in the network. LDP peers are two MPLS routers that use LDP to exchange label/FEC mapping information. An LDP session is created between LDP peers. A single LDP session allows each peer to learn the other's label mappings and to distribute its own label information (LDP is bidirectional), and exchange label binding information.
LDP signaling works with the MPLS label manager to manage the relationships between labels and the corresponding FEC. For service-based FECs, LDP works in tandem with the Service Manager to identify the virtual leased lines (VLLs) and pseudowires (PWs) to signal.
An MPLS label identifies a set of actions that the forwarding plane performs on an incoming packet before discarding it. The FEC is identified through the signaling protocol (in this case LDP), and is allocated a label. The mapping between the label and the FEC is communicated to the forwarding plane. In order for this processing on the packet to occur at high speeds, optimized tables that enable fast access and packet identification are maintained in the forwarding plane.
After an unlabeled packet ingresses the 7705 SAR, classification policies associate it with a FEC, the appropriate label is imposed on the packet, and then the packet is forwarded. Other actions can also take place on a packet before it is forwarded, including imposing additional labels, other encapsulations, or learning actions. After all actions associated with the packet are completed, the packet is forwarded.
When a labeled packet ingresses the router, the label or stack of labels indicates the set of actions associated with the FEC for that label or label stack. The actions are performed on the packet and then the packet is forwarded.
The LDP implementation provides support for DU, ordered control, and liberal label retention mode.
For LDP label advertisement, DU mode is supported. To prevent filling the uplink bandwidth with unassigned label information, Ordered Label Distribution Control mode is supported.
A PW/VLL label can be dynamically assigned by targeted LDP operations. Targeted LDP allows the inner labels (that is, the VLL labels) in the MPLS headers to be managed automatically. This makes it easier for operators to manage the VLL connections. There is, however, additional signaling and processing overhead associated with this targeted LDP dynamic label assignment.