Ldp - HP 10500 SERIES Configuration Manual

Upon receiving the labeled packet, Router C looks for the ILM entry that contains the label 40 to
2.
get the Token value. Because the Token value is not empty, Router C looks for the NHLFE entry
containing the Token value. According to the NHLFE entry, Router C swaps the original label with
label 50, and then forwards the labeled packet to the next hop LSR (Router D) through the outgoing
interface (GigabitEthernet 1/0/2).
Upon receiving the labeled packet, Router D (the egress) looks for the ILM entry according to the
3.
label (50) to get the Token value. Because the Token is empty, Router D removes the label from the
packet. If the ILM entry records the outgoing interface, Router D forwards the packet through the
outgoing interface. If no outgoing interface is recorded, router D forwards the packet according to
the IP header of the packet.
PHP
In an MPLS network, when an egress node receives a labeled packet, it looks up the LFIB, pops the label
of the packet, and then performs the next level label forwarding or performs IP forwarding. The egress
node needs to do two forwarding table lookups to forward a packet: looking up the LFIB twice or looking
up the LFIB and the FIB once each.
The penultimate hop popping (PHP) feature can pop the label at the penultimate node to relieve the
egress of the label operation burden.
PHP is configured on the egress node. The label assigned by a PHP-capable egress node to the
penultimate hop can be one of the two listed:
• IPv4 explicit null label 0—The egress assigns an IPv4 explicit null label to a FEC and advertises the
FEC-label binding to the upstream LSR. When forwarding an MPLS packet, the upstream LSR
replaces the label at the stack top with the explicit null label and then sends the packet to the egress.
When the egress receives the packet, which carries a label of 0, it does not look up for the LFIB entry
but pops the label stack directly and performs IPv4 forwarding.
Implicit null label 3—This label never appears in the label stack. An LSR directly performs a pop
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operation to the labeled packets that match the implicit null label rather than substituting the implicit
null label for the original label at the stack top. After that, the LSR forwards the packet to the
downstream egress LSR. The egress directly performs the next level forwarding upon receiving the
packet.

LDP

LDP establishes LSPs dynamically. Using LDP, LSRs can map network layer routing information to data link
layer switching paths.
Basic concepts of LDP
LDP session—LDP sessions are established between LSRs based on TCP connections and used to
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exchange messages for label binding, label releasing, and error notification.
• LDP peer—Two LSRs using LDP to exchange FEC-label bindings are LDP peers.
LDP message type
LDP messages fall into the following types:
Discovery messages—Declare and maintain the presence of LSRs.
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Session messages—Establish, maintain, and terminate sessions between LDP peers.
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• Advertisement messages—Create, alter, and remove FEC-label bindings.
Notification messages—Provide advisory information and notify errors.
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