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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.

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