3Com 8807 Configuration Manual page 509

Figure 121 Label position in packet
Ethernet Ethernet
/SONET /SONET /SONET /SONET
packet packet Ethernet header/ PPP header Ethernet header/ PPP header
/ SDH / SDH / SDH / SDH
Frame mode Frame mode
ATM ATM packet packet ATM header ATM header
Cell mode Cell mode
ATM packet ATM 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.
/PPP /PPP /PPP /PPP
PPP PPP
ATM ATM ATM ATM ATM ATM
VPE/VCI VPI/VCI VPE/VCI VPE/VCI VPI/VCI VPI/VCI VPI/VCI VPE/VCI
MPLS Basic Concepts
flag flag Layer 3 data Layer 3 data
flag flag Layer 3 data Layer 3 data
flag flag Layer 3 data Layer 3 data
509