R3303-HP HSR6800 Routers MPLS Configuration Guide

Figure 6 Independent label distribution control mode

• In ordered mode, an LSR distributes its label binding for a FEC upstream only when it receives a

label binding for the FEC from its downstream or it is the egress of the FEC. In Figure 5, la

bel

distribution control is in ordered mode. If the label advertisement mode is DU, an LSR distributes a

label upstream only when it receives a label binding for the FEC from its downstream. If the label

advertisement mode is DoD, after an LSR (Transit in this example) receives a label request from its

upstream (Ingress), the LSR (Transit) sends a label request to its downstream (Egress). Then, after the

LSR (Transit) receives the label binding from its downstream (Egress), it distributes a label binding to

the upstream (Ingress).

Label retention modes include the liberal mode and the conservative mode.

• In liberal mode, an LSR keeps any received label binding regardless of whether the binding is from

the next hop for the FEC or not. This mode allows for quicker adaptation to route changes but

wastes label resources because LSRs keep extra labels. The device supports only the liberal mode.

• In conservative mode, an LSR keeps only label bindings that are from the next hops for the FECs.

This mode allows LSRs to maintain fewer labels but makes LSRs slower in adapting to route

changes.

MPLS forwarding

LFIB

An LFIB comprises the following table entries:

• Next Hop Label Forwarding Entry—NHLFE describes the label operation to be performed. It is used

to forward MPLS packets.

• FEC to NHLFE map—FTN maps each FEC to a set of NHLFEs at the ingress LSR. The FTN map is

used for forwarding unlabeled packets that need MPLS forwarding. When an LSR receives an

unlabeled packet, it looks for the corresponding FIB entry. If the Token value of the FIB entry is not

Invalid, the packet must be forwarded through MPLS. The LSR then looks for the corresponding

NHLFE entry according to the Token value to determine the label operation to be performed.