The length field is 6
bits and is only valid if the frame length is less than 64 octets. Since this is greater
than the minimum Ethernet payload length, this enables transport of small frames
over Ethernet links. These bits are, of course, not used for Ethernet pseudowires
since a minimum-sized Ethernet frame (the smallest frame the pseudowire will
carry) is bigger than the minimum Ethernet payload length!
?– The 16-bit sequence number may be useful when carrying Layer 2 protocols such
as ATM and Frame Relay, which have strict requirements that frame order be
maintained over a network that may reorder packets. It is rarely implemented
in the Ethernet pseudowire case because most Ethernet applications do not have
strict frame order requirements; IP/MPLS networks do not generally reorder packets,
and many routers have forwarding plane implementations that are unable to
support sequence number processing.
Given all the above, it is not surprising that the control word is rarely used for
Ethernet pseudowires.
Establishing Pseudowires Pseudowires may be established either using static configuration
??”where an operator configures the labels to be used to identify the pseudowire
on the PEs at either end of the pseudowire and configures each PE to push the label
expected by the other onto the payload before adding the tunnel label??”or using LDP
signalling between the PEs at either end of the pseudowire.
Pages:
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989