H3C Technologies H3C SR8800 - Manual (Page 19)

8
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. An egress
node needs to do forwarding table lookup twice to forward a packet: looks up the LFIB twice, or looks
up the LFIB once and the FIB once.
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:
• A label value of 0 represents an IPv4 explicit null label. 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 stack directly and performs IPv4 forwarding.
• 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. Then, it forwards the packet
to 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
• 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.
• LDP peer
Two LSRs with an LDP session established between them and 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 on a network.
• Session messages—Establish, maintain, and terminate sessions between LDP peers.
• Advertisement messages—Create, alter, or remove FEC-label bindings.
• Notification messages—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:
1. Discovery