Many PE devices support 802.1ad provider bridging
(or ???Q-in-Q???) on their access ports, enabling carriers to assign a unique Provider
VLAN to each customer-facing Layer 2 switch port and then to support multiple
Customer VLANs within each of those ports. Thus, customer VLAN provisioning
can be constrained to the PE device.
?– MAC address scaling In a Layer 2 switched network, each switch learns the
MAC addresses of all active stations on all VLANs that pass through that switch.
With Ethernet pseudowires PEs do not have to learn MAC addresses, whereas for
VPLS MAC address knowledge is constrained to the PE devices that are members
of each VPLS instance (hiding MAC addresses from core LSRs). MPLS PE devices
also typically support larger numbers of MAC addresses than Layer 2 Ethernet
switches. This scaling is critical in cases where carriers connect customer switches
directly to a Layer 2 service. The N-PE devices in H-VPLS will typically learn more
MAC addresses than the U-PEs, and for this reason, there has been some investigation
of solutions that remove the requirement for the N-PEs to learn customer
MAC addresses.
?– Use of facilities Layer 2 Ethernet switches are interconnected using Ethernet
links.
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