H3C Technologies H3C S10500 Series Switches - Manual (Page 65)

54
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: looks up the LFIB twice, or looks up
the LFIB once and the FIB once.
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 following:
• 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. The upstream LSR directly performs
a pop operation to the labeled packets that match the implicit null label rather than substitutes the
implicit null label for the original label at the stack top, and forwards the packet to the downstream
egress LSR. The egress directly performs the next level forwarding upon receiving the packet.
LDP
LDP 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 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.
• Session messages—establish, maintain, and terminate sessions between LDP peers.
• Advertisement messages—create, alter, and remove FEC-label bindings.
• Notification messages—provide advisory information and notify errors.
LDP session, advertisement, and notification messages use TCP for reliability. Discovery messages use
UDP for efficiency.
LDP operation
LDP goes through the following phases in operation:
1. Discovery
Each LSR sends hello messages periodically to notify neighboring LSRs of its presence. In this way, LSRs
can automatically discover their LDP peers. LDP provides the following discovery mechanisms: