Ldp Overview; Ldp Basic Concepts - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide
On the LSR along the LSP, the mapping table of the import/export labels has been
established (the element of this table is referred to as Next Hop Label Forwarding Entry
(NHLFE)). When the labeled packet arrives, LSR only needs to find the corresponding
NHLFE from the table according to the label and replace the original label with the new
special label, and then forwards the labeled packet. This process is called Incoming
Label Map (ILM).
On the Ingress, MPLS specifies FEC of specific packet, and the following routers only
need to forward it by label switching therefore this method is much simpler than the
routine network layer forwarding.
Note:
TTL Processing:
For labeled packet, it is necessary to copy the TTL value in the original IP packet into
the TTL field in the label. While forwarding the label type packet, LSR will perform
minus one operation for the TTL field of the label on the top of the stack. When the label
is out of the stack, the TTL value on the top of the stack is copied back to IP packet or
the label of lower layer.
However, while LSP goes through the non-TTL LSP segment composing of ATM-LSR
or FR-LSR, the LSR inside the non-TTL LSP segment is not capable of processing TTL
field. In this case, it is necessary to carry out unified processing for TTL while entering
non-TTL LSP segment, namely, to reduce for one time the value that reflects the length
of this non-TTL LSP.

1.4 LDP Overview

The LDP is responsible for message regulation and relevant processing in the label
distribution.
An LSR can directly map the routing information on the network layer to a switch path
on the data link over LDP, and then establish an LSP on the network layer. The LSP can
be set up between two adjacent LSRs or terminated at an Egress LSR. All the LSRs in
between adopt the label switching.

1.4.1 LDP Basic Concepts

I. LDP Peers
LDP peers refer to two LSRs undergo an LDP session by exchanging label/FEC
mapping information over LDP.
The LDP peers can obtain the other's label information through an LDP session,
namely, the LDP is bidirectional.
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Chapter 1 MPLS Architecture