When used to create LSP tunnels, Label Distribution Protocol (LDP) allows a set of destination IP prefixes (known as a Forwarding Equivalence Class or FEC) to be associated with an LSP.
Each Link State Router (LSR) establishes a peer relationship with the neighboring LDP-enabled routers and exchanges label mapping information. This label mapping information is stored in an LDP database on each LSR. When an LSR determines that one of the peers is the next-hop for a FEC, the LSR uses the label mapping information from the peer to set up an LSP that is associated with the FEC.
The devices advertise their loopback addresses to their LDP peers as a 32-bit prefix-type FEC. When an LSR installs a label for a FEC, it also creates an MPLS tunnel route, which is then made available to routing applications. This allows each router to potentially be an ingress LER for an LSP whose destination is the device's loopback address.
The result of an LDP configuration is a full mesh of LSPs in an MPLS network, with each LDP-enabled router a potential ingress, transit, or egress LSR, depending on the destination.
The system supports LDP for the configuration of non-traffic-engineered tunnel LSPs in an MPLS network. LDP is described in RFC 5036.
The vRouter implementation supports the following aspects of LDP:
- Liberal label retention—Each LSR sends its peers Label Mapping messages, which map a label to a FEC. The peer LSR receiving these messages retain all of the mappings, even though they may not actually be used for data forwarding.
- Unsolicited label advertisement—The LSR sends Label Mapping messages to its LDP peers even though they did not explicitly request them.
- Ordered label distribution—The LSR sends a Label Mapping message to its peers only when it knows the next hop for a FEC, or is itself an egress LER for the FEC. When an LSR does not know the next hop for a FEC, and is not an egress LER for the FEC, it waits until a downstream LSR sends it a Label Mapping message for the FEC. At this point, the LSR can send Label Mapping messages for the FEC to its peers. This allows label mappings to be distributed, in an orderly fashion, starting from the egress LER and progressing upstream.
The LDP label space ID has a default value of zero which improves interoperability with routers from other vendors.