3Com Switch 8800 Family Configuration Guide

MPLS Basic Concepts 509
Figure 121 Label position in packet
In Ethernet packets and PPP packets, label stack lies between layer 2 header and
layer 3 data, acting like a shim.
3 Label assignment and distribution
Label distribution refers to the process of creating a corresponding label switching
path (LSP) for a FEC.
In the MPLS architecture, the decision to bind a particular label to a particular FEC
is made by downstream LSR; after making the decision, the downstream LSR
notifies the upstream LSR. That is to say, the label is assigned by the downstream
LSR, and the assigned label is distributed from downstream to upstream.
Two label distribution modes are available in MPLS: downstream unsolicited (DU)
mode and downstream on demand (DoD) mode.
■ For a specific FEC, if LSR originates label assignment and distribution even
without receiving label request message from upstream, it is in DU mode.
■ For a specific FEC, if LSR begins label assignment and distribution only after
receiving label request message from upstream, it is in DoD mode.
The upstream and downstream which have adjacency relation in-label distribution
should reach agreement on label distribution mode.
To distribute labels to its peer, the LSR can use Label Distribution Protocol (LDP)
messages or make the labels carried on other routing protocol messages.
n
Upstream and downstream are just on a relative basis: For a packet forwarding
process, the transmit router serves as upstream LSR and receive router serves as
downstream LSR. Currently, the Switch 8800 Family series adopt the DU label
distribution mode.
4 Label assignment control mode
There are two modes to control the assignment and distribution of labels:
independent mode and ordered mode.
In independent control mode, each LSR can send label mapping messages to the
LSRs it connects to at anytime.
In ordered control mode, a LSR can send label mapping messages to upstream
only when it receives a specific label mapping messages of the next hop of a FEC
or the LSR serves as LSP (Label Switching Path) egress node.
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Frame mode
packet
Cell mode
Ethernet header/ PPP header flag Layer 3 data
ATM header
flag Layer 3 data
VPI/VCI
flag Layer 3 data
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packet
Frame mode
packet
Cell mode
Ethernet header/ PPP header flag Layer 3 data
ATM header
flag Layer 3 data
VPI/VCI
flag Layer 3 data