
5
•
In independent mode, an LSR can distribute label bindings upstream at anytime. This means
that an LSR might have distributed a label binding for a FEC to its upstream LSR before it
receives a binding for that FEC from its downstream LSR. As shown in
, in independent
label distribution control mode, if the label advertisement mode is DU, an LSR assigns labels to
its upstream even if it has not obtained labels from its downstream. If the label advertisement
mode is DoD, the LSR distributes a label to its upstream as long as it receives a label request
from the upstream.
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
, label
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