Ethernet over MPLS starts with a connectionless IP network. MPLS creates point-topoint
connections over this connectionless network. Ethernet over MPLS then emulates
a connectionless network using the MPLS connections. Running two network emulations
in order to get back to where you started is inherently inefficient. If Ethernet
services are a small part of the services offered by a large MPLS network, this makes
sense. If one is starting from scratch to create a network offering primarily Ethernet
services, bridges can do a more efficient job.
Similarly, SONET, ATM, and other circuit-switching technologies are reliable, proven
ways to carry point-to-point and point-to-multipoint traffic, including Ethernet. If only
E-Line or single-point-to-multipoint EVCs are to be offered, these technologies are
perfectly adequate. But like MPLS, when using circuit technology to emulate a shared
medium technology, you wind up with an inherently inefficient emulation.
Bridges have limits, of course. The power of bridges lies in their ability to provide
millions of independent EVCs, each of which can reasonably connect a modest number
of endpoints.
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