6 Broadcasting of frames is a regular part of an Ethernet learning bridge??™s operation. Consider a network
of six learning bridges. If point-to-point links (e.g., using EoS transport) interconnect those bridges, then
each bridge must have five bridge ports for the bridge interconnection and must replicate a broadcast frame
five times to ensure that each of the other bridges receives the broadcast frame. If a shared LAN connects
the six bridges (e.g., using Ethernet/RPR/SONET), then each bridge has only one bridge port for the bridge
interconnection and must send only a single broadcast frame over that bridge port.
SONET/MSPP 319
The pseudowire is the lens that has given clarity to this view. In standard terms, a
pseudowire is a ???mechanism that emulates the essential attributes of a service such as
ATM, frame relay, or Ethernet over a packet switched network??? [15]. In other words, it
is an adaptation of a packet-based service that makes an IP/MPLS network appear to
be a wire??”a pseudowire??”for that service.
This process7 is called PseudoWire Emulation Edge-to-Edge (PWE3), and for Ethernet
transport, it is straightforward: An ingress provider edge (PE) device adapts an Ethernet
frame into a pseudowire by adding additional header information (a pseudowire label
or ???shim??? header).
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