It depends. I've most commonly seen some type of Ethernet-based handoff, but it isn't necessarily so. It could come in on a serial line, like a T1, or a DS3 speed HSSI interface, or just about anything else. In practice, the interface speed and type of connection are typically specified when ordering. I almost always see fiber coming into enterprise-class customers, and if there's a copper handoff to the customer it's usually based on a managed, co-located, fiber/copper conversion box. My normal expectation nowadays is that it will be some kind of optical Ethernet. At 100 Mbps and below I've often seen it delivered on copper FastEthernet (fiber into the building, with some kind of converter or provider-managed switch for the copper).
Here are samples of some of the MPLS connections I've worked on recently (with provisioned MPLS speeds of 3 Mbps up to 1 Gbps):
- FastEthernet0/1, Full-duplex, 100Mb/s, 100BaseTX/FX
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hssi1/0, Hardware is M1T-HSSI-B
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GigabitEthernet0/0/0, Full-duplex, 1000Mb/s, link type is autonegotiation, media type is LX
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GigabitEthernet0/0/0, Full Duplex, 100Mbps, link type is force-up, media type is RJ45
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GigabitEthernet0/1, Full-duplex, 100Mb/s, media type is RJ45
The layer two handoff is independent of the fact that you're using MPLS for a private L3 network. Fiber-based Ethernet is common at higher speeds. Copper based Ethernet is more common for lower speeds (and typically for very short distances, within the same building).
MPLS backbones typically use a high speed type of Ethernet over fiber (often running through DWDM) or even SONET connectivity. Fundamentally, the hardware used is often mostly the same as it would be in a purely IP-based network with a few extra configuration commands to provide the MPLS L3 VPN-based isolation required.
