3Com S7906E - Switch, Configuration Manual

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with MPLS echo replies. If the ping initiator receives the replies, the LSP is considered perfect for
forwarding data.
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MPLS LSP traceroute is a tool for locating LSP errors. By sending MPLS echo requests to the
control plane of each transit LSR, it can determine whether the LSR is really a transit node on the
LSP.
The destination address in the IP header of an MPLS echo request is set to an address on 127.0.0.0/8
(a loopback address of the LSR) and the TTL is set to 1, so as to prevent further forwarding of the
request when the request reaches the egress.
LDP Overview
Basic Concepts of LDP
The LDP protocol dictates the messages to be used in label distribution and the related processes.
Using LDP, LSRs can map network layer routing information to data link layer switching paths directly
and further establish LSPs. LSPs can be established between both neighboring LSRs and LSRs that
are not directly connected, making label switching possible at all transit nodes on the network.
For detailed description about LDP, refer to RFC 3036 “LDP Specification”.
LDP peer
Two LSRs with an LDP session established between them and using LDP to exchange label bindings
are called LDP peers, each of which obtains the label bindings of its peer over the LDP session between
them.
LDP session
LDP sessions are used to exchange messages for label binding and releasing.
LDP sessions come in two categories:
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Local LDP session: Established between two directly connected LSRs.
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Remote LDP session: Established between two indirectly connected LSRs.
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.