Ldp - HP 12500 Series Configuration Manual

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 four phases in operation:
Discovery
1.
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 two discovery mechanisms:
Basic discovery mechanism—Discovers directly connected LSRs. An LSR periodically sends LDP
link Hello messages to multicast address 224.0.0.2 that identifies all routers on the subnet to
advertise its presence.
Extended discovery mechanism—Discovers indirectly connected LDP peers. An LSR
periodically sends LDP Hello messages to a given IP address so that the LSR with the IP address
can discover the LDP peer.
Session establishment and maintenance
2.
After an LSR finds a LDP peer, they start to establish a session through the following two steps:
Establish a TCP connection between them.
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