Penultimate Hop Popping; Figure 10 Generating Alsp - Zte ZXR10 8900 Series User Manual

ZXR10 8900 Series User Manual (MPLS Volume)
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When RA receives binding information, RA acts in the same way
as RB does.
LIB is always isochronous with routing tables. Once a new non-
BGP route is generated in the routing table, LSR distributes a la-
bel for this route. LSR does not distribute labels for BGP routes.
Instead, it distributes a label for the next hop of a BGP route.
After the interaction of labels finishes, a LSP forms. When LSRs
forward messages, they forward them according to labels instead
of looking up routing tables, as shown in
LSP
When RA receives a message with destination address 47.1.1.1, it
looks up its routing table first, and then it looks up label forwarding
table. When it finds that the FEC 47.1.0.0/16 corresponds to the
label "50", RA adds the label to the header of the message, and
sends it from interface 2.
When RB receives the message with label "50" from interface 1, it
looks up the label forwarding table, then RA changes the label to
"40" and sends the message from interface 2.
When RC receives the message with label "40" from interface 1, it
looks up the label forwarding table. RC finds that the destination
of the message is a network segment connecting to itself directly.
Therefore, RC removes the label and sends the IP message.

Penultimate Hop Popping

In a MPLS domain, when the egress LER receives message from its
neighbor, it looks up label forwarding table first and removes the
label, then it looks up routing table and forwards the IP message.
This decreases the performance of the LER and increases forward-
ing complexity. Penultimate hop popping mechanism solves this
problem.
By default, LDP enables only penultimate hop popping mechanism
for direct routes and aggregated routes. For direct routes, LER
has to look up routing table to get next hop information before it
Figure 10.