Juniper JUNOSE, Configuration Manual

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For information about collecting statistics for the MPLS shim interfaces, see
Monitoring Layer 2 Services over MPLS on page 557
Multiple Layer 2 Services over MPLS
When you configure an MPLS shim interface over an ATM, Frame Relay, or HDLC
layer 2 interface, no other interface (for example, PPP or IP) can be stacked above
the layer 2 interface.
By contrast, when you configure an MPLS shim interface over any Ethernet or
Ethernet/VLAN interface, both the MPLS shim interface and other interfaces (such
as IP, PPP, or PPPoE) can be stacked above the layer 2 interface.
When you configure both an MPLS shim interface and another interface over a layer 2
interface, traffic for a protocol explicitly configured in the interface stack is forwarded
to that protocol layer for further processing. Traffic for any nonconfigured protocols
is forwarded to the MPLS shim interface on the other side of the connection.
When the MPLS shim interface is the only layer stacked above the layer 2 interface,
as is the case with ATM, Frame Relay, and HDLC, then all traffic is forwarded to the
MPLS shim interface and across the MPLS tunnel.
ATM Layer 2 Services over MPLS
ATM layer 2 services over MPLS provide ATM switch-like functionality for E Series
routers. This feature is useful for customers who run IP in the majority of their network
but still have to carry a small amount of non-IP traffic, as in the example shown in
Figure 115 on page 509.
Figure 115: Common ISP Network
In this scenario, it is not economical to have special ATM switches in front of E Series
routers just to accommodate the small percentage of non-IP traffic. The ATM layer
2 services over MPLS feature lets you replace the ATM traffic selector switch with an
E Series router, as shown in Figure 116 on page 510. The two routers pass traffic
between two interfaces through an MPLS tunnel using Martini encapsulation,
regardless of the contents of the packets.
Multiple Layer 2 Services over MPLS
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Chapter 6: Layer 2 Services over MPLS Overview