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DES-7200 Configuration Guide Chapter 1 MPLS Configuration
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7. If the label is popped out on the PHP LSR, the last-hop egress LER receives the original
IP packet and forwards it according to the IP routing table.
1.1.6 LSP Setup and Loop Detection
The pseudo MPLS wire is an LSP. One FEC data stream is assigned with different labels on
different MPLS nodes and forwarded according to the labels. The path that the data stream
travels is an LSP that consists of a series of LSRs. The data streams of the same FEC pass
through the same LSP.
LSP Setup
LSP Loop Control
1.1.6.1 LSP Setup
The LSP setup is in fact the process of binding the FEC to a label and notifying adjacent LSRs of
the binding. This process is completed by the LDP. RFC 3036 stipulates the protocol
specifications of LDP, the interactive process of LSRs, and the message formats.
The LDP detects adjacent LSRs by periodically sending Hello packets. The LDP Hello
packets adopt UDP encapsulation and use the well-known port 646 as the destination port.
The destination address of these packets is the multicast address of all routers in the subnet
(the corresponding IP address is 224.0.0.2). Upon the discovery of a neighboring LSR, the
LDP session is triggered. Setting up an LDP session involves two steps:
1) Set up a transmission connection. This is in fact the completion of TCP three-way
handshakes that do not require any interaction of LDP messages.
2) Initialize the session. The LDP session parameters are negotiated and determined by
exchanging the initialization information of both parties, such as the label distribution mode,
Keepalive duration, and the maximum length of Protocol Data Unit (PDU).
After the LDP session is created and both parties enter the Operational status, the two parties
can exchange label messages to distribute and manage labels, and create an LSP for each FEC.
During the LSP setup process, there are two label distribution modes: Downstream on Demand
(DOD) and Downstream Unsolicited (DU). In DOD mode, one LSR responds to a label binding
message only after the receipt of a label request from an adjacent LSR. In DU mode, one LSR
voluntarily sends label binding messages to its adjacent LSRs without receiving any request.
During the LSP setup process, there are two label control methods: independent and ordered
control. In independent control mode, each LSR announces to its adjacent devices the binding of
labels and FECs at any required time. In independent DOD mode, one LSR can immediately
answer an upstream label mapping request without waiting for the label mapping from the next