Juniper JUNOSE, Configuration Manual

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You can configure an HDLC layer 2 circuit between two serial interfaces, between
two packet over SONET (POS) interfaces, or between a serial interface and a POS
interface. The interfaces at either end of the circuit can operate at the same speed
or at different speeds. For example, you can configure an HDLC layer 2 circuit
between a serial interface on a T1 circuit and a POS interface on an OC3 circuit.
Interface Stacking
When you configure an MPLS shim interface above an HDLC layer 2 interface, which
is in turn stacked above a serial or POS interface, no other interfaces (for example,
PPP) can be stacked above the HDLC interface. In other words, the HDLC interface
can have only one upper interface.
In practice, this means that you cannot issue the mpls-relay command (to create
the HDLC layer 2 circuit) and then issue an encapsulation command (such as
encapsulation ppp ) for the same HDLC interface. If you attempt to do so, the router
prevents the configuration and displays an error message.
This behavior contrasts with that of bridged Ethernet and Ethernet interfaces (with
and without VLANs), which allow configuration of both an MPLS shim interface and
another interface (such as IP, PPP, or PPPoE) above the layer 2 interface.
Encapsulation
The JUNOSe implementation of HDLC layer 2 circuits supports encapsulation of either
HDLC frames or PPP frames within MPLS frames. By default, the router uses VC-type
HDLC signaling and HDLC encapsulation to encapsulate HDLC frames in MPLS.
However, if you want the router to encapsulate PPP frames directly in MPLS without
the HDLC header, you can include the optional
relay-format ppp keywords in the
mpls-relay or route interface command to cause the router to use VC-type PPP
signaling and PPP encapsulation instead of the default VC-type HDLC signaling and
HDLC encapsulation. This option, which is available only for serial and POS interfaces,
is useful if the traffic carried on the serial or POS interface contains actual PPP packets
and not, for example, Cisco HDLC packets.
Control Word Support
The router always sends a control word for HDLC layer 2 circuits, regardless of
whether or not sequencing is enabled.
Local Cross-Connects
You can configure an HDLC layer 2 circuit in a local cross-connect configuration
between serial or POS interfaces within the same router. In this configuration, the
pairs of HDLC interfaces are directly cross-connected to each other. The
cross-connected interfaces can be different types and operate at different speeds;
for example, you can cross-connect a serial interface on a T1 circuit and a POS
interface on an OC3 circuit.
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HDLC Layer 2 Services over MPLS
JUNOSe 11.0.x BGP and MPLS Configuration Guide