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3Com Switch 8800 Configuration Guide Chapter 35 MPLS Architecture
35-3
Ethernet
SONET/SDH
packet
Ethernet
header/PPP header
Flag Layer 3 data
Fram e mode
ATM packet
Cell mode
ATM packet
ATM header Flag Layer 3 data
VPI/VCI Layer 3 data
Ethernet
SONET/SDH
packet
Ethernet
header/PPP header
Flag Layer 3 data
Fram e mode
ATM packet
Cell mode
ATM packet
ATM header Flag Layer 3 data
VPI/VCI Layer 3 data
Figure 35-2 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. In ATM cell mode packets, VPI/VCI is used as the label.
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.
z For a specific FEC, if LSR originates label assignment and distribution even
without receiving label request message from upstream, it is in DU mode.
z 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.
Note:
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 adopts 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.