Layer 2 Circuit Transport Mode on ATM MICs Overview
On MX Series routers with ATM MICs, you can configure Layer 2 circuit cell relay or Layer 2 circuit ATM Adaptation Layer 5 (AAL5). Layer 2 circuit cell relay and Layer 2 circuit AAL5 are defined in Encapsulation Methods for Transport of Asynchronous Transfer Mode (ATM) Over MPLS Networks (RFC 4717). Layer 2 circuit cell-relay and Layer 2 circuit AAL5 transport modes allow you to send ATM cells between ATM interfaces across a Layer 2 circuit-enabled network. Layer 2 circuits are designed to transport Layer 2 frames between provider edge (PE) routers across an MPLS backbone.
Layer 2 circuit cell-relay and Layer 2 circuit AAL5 transport modes accept a stream of ATM cells, convert these to an encapsulated Layer 2 format, and then tunnel them over an MPLS or IP backbone, where a similarly configured router segments these packets back into a stream of ATM cells, to be forwarded to the virtual circuit configured for the far-end router. In Layer 2 circuit cell-relay transport mode, ATM cells are bundled together and transported in packet form to the far-end router, where they are segmented back into individual ATM cells and forwarded to the ATM virtual circuit configured for the far-end router. You use Layer 2 circuit AAL5 transport mode to send AAL5 segmentation and reassembly protocol data units (SAR-PDUs) over the Layer 2 circuit.