H3C Technologies H3C S12500-X Series Switches - Manual (Page 29)

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Label distribution control
LDP controls label distribution in one of the following ways:
• Independent label distribution—Distributes a FEC-label mapping to an upstream LSR at any time.
An LSR might distribute a mapping for a FEC to its upstream LSR before it receives a label mapping
for that FEC from its downstream LSR. As shown in
Figure 11 , in DU mode, each LSR distributes a
label mapping for a FEC to its upstream LSR whenever it is ready to label-switch the FEC, without
waiting for a label mapping for the FEC from its downstream LSR. In DoD mode, an LSR distributes
a label mapping for a FEC to its upstream LSR after it receives a label request for the FEC, without
waiting for a label mapping for the FEC from its downstream LSR.
Figure 11 Independent label distribution control mode
• Ordered label distribution—Distributes a label mapping for a FEC to its upstream LSR only after it
receives a label mapping for that FEC from its downstream LSR unless the local node is the egress
node of the FEC. As shown in
Figure 10 , in DU mode, an LSR distributes a label mapping for a FEC
to its upstream LSR only if it receives a label mapping for the FEC from its downstream LSR. In DoD
mode, when an LSR (Transit) receives a label request for a FEC from its upstream LSR (Ingress), it
continues to send a label request for the FEC to its downstream LSR (Egress). After the transit LSR
receives a label mapping for the FEC from the egress LSR, it distributes a label mapping for the FEC
to the ingress.
Label retention mode
The label retention mode specifies whether an LSR maintains a label mapping for a FEC learned from a
neighbor that is not its next hop.
• Liberal label retention—Retains a received label mapping for a FEC regardless of whether the
advertising LSR is the next hop of the FEC. This mechanism allows for quicker adaptation to
topology changes, but it wastes system resources because LDP has to keep useless labels. The
device only supports liberal label retention.
• Conservative label retention—Retains a received label mapping for a FEC only when the
advertising LSR is the next hop of the FEC. This mechanism saves label resources, but it cannot
quickly adapt to topology changes.