# 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 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.

#### Related Documentation

- Configuring HDLC Layer 2 Services