MPLS uses the label distribution protocols to create and maintain the association of labels, which creates LSP dynamically, and the association of it with a particular destination. These protocols are in charge of signaling the LSP path and inform the labels that are used among the devices, from the ingress router to the egress router. A label distribution protocol deals with the negotiation of labels, which will be used in each link to switch traffic according to its destination.
There are two main label distribution protocols used in MPLS environments: Label Distribution Protocol (LDP) and the Resource Reservation Protocol (RSVP). RSVP is a generic protocol for the reservation of resources, which was adopted to be used in MPLS environments. LDP is a protocol specifically designed for MPLS environments.
RSVP is a protocol, which was redesigned in order to include the ability to transport opaque objects. These objects have no particular meaning for RSVP and are transported among devices for some other protocol (like MPLS) to use it, which helps to extend the functionalities of MPLS. These objects make it possible to distribute and maintain an information database and the inherent resources reservation to a protocol. The reservation and distribution of labels, and most of MPLS functionalities use opaque objects. The main advantage that the use of the RSVP protocol has is the possibility of implementing traffic engineering on MPLS networks (indicate in which way the LSPs paths are established, following different guidelines to the IGP metric) and the possibility of implementing redundancy in the LSPs.
The RSVP protocol, in its extended version, allows:
- To distribute information of labels association between the Label Switching Routers (LSR).
- To provide LSP paths which support explicit routes (indicating to the LSP over which nodes it has to be established, ignoring the IGP metric).
- To create path registers (nodes) which the LSP has used in the establishment of the path.
- To reserve resources in the routers that cover the LSP
- To implement keep-alive mechanisms in the LSPs (indicate if the LSP is active and can be used).
- To implement redundancy schemes (Creation of primary and secondary LSP paths and Fast-Reroute techniques in case of failure).
The LDP protocol associates a group of destination prefixes with a LSP in particular. A set of destination prefixes are known as Forwarding Equivalence Class (FEC). All the destination prefixes of the FEC share an exit node in common and a routing path. The LDP protocol establishes LSP paths based on the IGP metric used in the network, negotiating labels in each hop. This way, when a node has the network’s visibility, it is considered exit for it and it starts a label signaling process. LDP is a simple protocol, ideal for networks which don’t require traffic engineering. This is due to the fact that the default behaviour is for the protocol to follow the IGP metric to build an LSP. The LDP presents the advantage of negotiating labels, and establish paths automatically, between LDP routers for the FEC destinations. This behavior makes the protocol easy to implement and configure but it is not very flexible or scalable.