Juniper JUNOSE 11.2.X BGP AND MPLS, Руководство по настройке

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HDLC Layer 2 Services over MPLS Overview
E Series routers support the creation of HDLC layer 2 circuits across an MPLS network.
An HDLC layer 2 circuit can carry any standard HDLC traffic (including PPP) or Cisco HDLC
traffic between two CE devices across an MPLS network. In an HDLC layer 2 circuit
configuration, an E Series router functions as one of the PE routers.
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
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JunosE 11.2.x BGP and MPLS Configuration Guide