The tunnel label may be removed before the packet reaches the
egress PE if PHP is used, and if the ingress and egress PE are adjacent, may never be
pushed on.
So when the ingress PE wants to send a frame along a pseudowire to the egress PE, it
first pushes on the PW label assigned by the egress PE and then (in the general case) adds
a tunnel label corresponding to the selected tunnel LSP and possibly additional labels.
MPLS 437
Any intermediate LSRs along the LSP need only examine the tunnel label in order to forward
the packet along the tunnel LSP??”they do not need to be aware of the existence of
one or more pseudowires within the tunnel LSP. Intermediate LSRs may also push or pop
additional labels, for example, for fast reroute protection or in cases where LSP hierarchy
is used. The egress PE uses the PW label to determine which attachment circuit to forward
the received Layer 2 frame on. Note that for some Layer 2 protocols, such as Ethernet (see
Figure 14.3 below), the frame header is carried across the pseudowire, whereas for others,
such as Frame Relay, it is removed and a new header is added by the egress PE.
NOTE When carrying Layer 2 traffic over MPLS, the MPLS label stack sits between
the service provider??™s Layer 2 header and the service user??™s Layer 2 frame (which may
or may not include a header).
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