In such cases, it may be
necessary to fragment the frames before transmission and to reassemble the fragments
at egress and hence to carry an indication of whether a packet contains a
fragment, and if so, whether it contains the final fragment of the frame. Two bits
are provided for this. Fragmentation is rarely implemented for Ethernet pseudowires.
Service providers generally ensure that the MTU of their LSPs is sufficient to
transport the largest frame size supported by their service.
Figure 14.3 Ethernet pseudowire encapsulation
Ethernet frame
(FCS removed)
L2
FCS
Layer 2
header
Tunnel
label
VC
label
438 Chapter 14
?– One potential issue in carrying Layer 2 protocols over MPLS is that if the payload
plus MPLS headers is smaller than the minimum Ethernet payload length
(46 bytes), then padding will be added to the packet when it crosses an Ethernet
segment on the LSP path. Because MPLS has no length field, this padding will be
retained until the packet reaches the egress PE. When carrying IP over MPLS, the
length field in the IP packet header can be used to determine how much padding
must be removed; however, most Layer 2 protocols have no length field and instead
rely on physical layer framing to detect the end of the frame.
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