When Ethernet is carried over MPLS and there is an
Ethernet link between two MPLS LSRs, there will be two MAC headers on each packet,
separated by an MPLS label stack.
The Control Word The Generic PW MPLS Control Word defined in RFC 4385 and
shown in Figure 14.4 is required for some pseudowire types, such as Frame Relay, and
is optional for others, including Ethernet. When used, it is inserted between the label
stack and the payload.
The fields in the control word are used as follows:
?– The first four bits of the control word are set to all zeroes. This enables core LSRs,
which perform ECMP by inspecting the packet to identify flows, to distinguish
pseudowire payloads from IP packets (where the first four bits will be 0100 for
IPv4 and 0110 for IPv6).
?– In cases where the Layer 2 header is removed, there may be a need to carry header
flags such as congestion notification to the egress PE. Four bits are provided for
this. These bits are not used for Ethernet pseudowires, as there are no flags in the
Ethernet header.
?– There may be cases where large frames need to be carried over LSPs with insufficiently
large Maximum Transmission Units (MTUs).
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