H3C Technologies H3C S7500E Series Switches - Manual (Page 51)

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In this case, the penultimate hop popping (PHP) feature can pop the label at the penultimate node,
relieving the egress of the label operation burden and improving the packet processing capability of
the MPLS network.
PHP is configurable on the egress node. The label assigned by a PHP-capable egress node to the
penultimate hop can be one of the following:
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A label value of 0 represents an IPv4 explicit null label and is valid only when it appears at the
bottom of a label stack. An 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
substitutes 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 directly and performs IPv4 forwarding.
z
A label value of 3 represents an implicit null label and never appears in the label stack. When an
LSR finds that it is assigned an implicit null label, it directly performs a pop operation, rather than
substitutes the implicit null label for the original label at the stack top, and then forwards the
packet to the downstream LSR, that is, the egress. The egress thus can directly perform the next
level forwarding upon receiving the packet.
LDP
The LDP protocol is used to establish LSPs dynamically. Using LDP, LSRs can map network layer
routing information to data link layer switching paths.
Basic concepts of LDP
z
LDP session
LDP sessions are established between LSRs based on TCP connections and used to exchange
messages for label binding, label releasing, and error notification.
z
LDP peer
Two LSRs with an LDP session established between them and using LDP to exchange FEC-label
bindings are called LDP peers.
LDP message type
There are four types of LDP messages:
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Discovery message: Used to declare and maintain the presence of LSRs on a network.
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Session message: Used to establish, maintain, and terminate sessions between LDP peers.
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Advertisement message: Used to create, alter, or remove FEC-label bindings.
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Notification message: Used to provide advisory information and to notify errors.
For reliable transport of LDP messages, TCP is used for LDP session messages, advertisement
messages, and notification messages. UDP is used only for discovery messages.
LDP operation
LDP goes through four phases in operation:
Discovery
Every LSR that wants to establish LDP sessions sends Hello messages periodically to notify
neighboring LSRs of its presence. In this way, LSRs can automatically discover their LDP peers. LDP
provides two discovery mechanisms: