1Q
VLAN tags, but if so, the tags will then pass through the egress PE without
modification.
In general, the tagged mode is used for VLAN-based services and the raw mode for
port-based services, though the raw mode may be used for VLAN-based services if the
tag is stripped at ingress and a new tag added at egress.
An Ethernet pseudowire emulates a point-to-point Ethernet LAN with two endstations.
It acts as a nonlearning bridge??”all packets received from the attachment
circuit at the ingress PE will be forwarded over the pseudowire and transmitted over
the attachment circuit at the egress PE. The one exception to this is 802.3x PAUSE
frames, which are processed locally by each PE (the 802.3x protocol works over short
time windows, and hence it is better to have the local PE buffer frames, if possible,
than to send the PAUSE frames over the network).
Multi-Segment Pseudowires One limitation of the pseudowire architecture is that PEs
must be connected over MPLS LSPs. This becomes a problem in three cases:
?– Very large networks where there are too many PE devices for them to be fully
meshed over MPLS LSPs and where RSVP-TE is used for LSP signalling meshes
of more than around 100 nodes.
Pages:
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998