A system and method for performing rate adaptation of constant bit rate (CBR) client data for transmission over a Metro Transport Network (MTN) by defining a plurality of pseudo-Ethernet packets at a source node, assembling a plurality of Generic Mapping Procedure (GMP) frames by mapping a plurality of blocks from a stream of encoded blocks of CBR client data, a plurality of pad blocks, and GMP overhead into consecutive pseudo-Ethernet packets of the plurality of pseudo-Ethernet packets, inserting a variable number of idle blocks between one or more of the consecutive pseudo-Ethernet packets and inserting an MTN path overhead (POH) frame that is aligned to the plurality of GMP frames to generate a plurality of rate adapted GMP frames for transmission over the MTN to an intermediate node or a sink node.