Ldp Basic Operation - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide
On an LSP, along the data transmission direction, neighboring LSRs are respectively
called as upstream LSR and downstream LSR. On LSP1 shown in Figure 1-5, LSR B is
the upstream LSR of LSR C.
Labels can be distributed in two modes: downstream on demand (DoD) and
downstream unsolicited (DU), depending on whether label mapping distribution is done
at the downstream with solicitation or without.
1) DoD mode
In DoD mode, the label is distributed in this way: the upstream LSR sends label request
message (containing FEC descriptive information) to the downstream LSR, and the
downstream LSR distributes label for this FEC, and then sends the bound label back to
the upstream LSR through label mapping message.
When the downstream LSR feeds back the label mapping message depends on
whether this LSR uses independent label control mode or sequential label control
mode. When the sequential label control mode is used by the downstream LSR, the
label mapping message is sent back to its upstream LSR if only it has received the label
mapping message from its downstream LSR. And when the independent label control
mode is used by the downstream LSR, it will send label mapping message to its
upstream LSR immediately, no matter if it has received the returned label mapping
message from its downstream LSR.
Usually, the upstream LSR selects the downstream LSR according to the information in
its routing table. In Figure 1-5, the sequential label control mode has been used by the
LSRs on the way along LSP1, and the independent label control mode has been used
by the LSRs on LSP2.
2) DU mode
In DU mode, the label is distributed in the following way: when LDP session is
established successfully, the downstream LSR will actively distribute label mapping
message to its upstream LSR. The upstream LSR saves the label mapping information
and processes the received label mapping information according to the routing table.

1.4.3 LDP Basic Operation

The basic LDP operation includes:
Discovery phase
Session establishment and maintenance
LSP setup and maintenance
Session termination
I. Discovery Phase
The originating LSR periodically sends a Hello message to its adjacent LSRs, notifying
them its peer information, so that the LSR can automatically find its LDP peer.
There are two types of LDP discovery mechanisms.
3Com Corporation
1-8
Chapter 1 MPLS Architecture