The transmitter elastic buffer is described in the following files:
<project_dir>/<project_name>/<project_name>.srcs/sources1/ip/ <component_name>/example_design/<component_name>_tx_elastic_buffer.v[hd]
When the GMII is used externally (as in this example design), the GMII transmit signals (inputs to the core from a remote Ethernet MAC at the other end of the interface) are synchronous to a clock, which is likely to be derived from a different clock source to the core. For this reason, GMII transmit signals must be transferred into the core main clock domain before they can be used by the core. This is achieved with the TX elastic buffer, an asynchronous FIFO implemented in distributed RAM. The operation of the elastic buffer is to attempt to maintain a constant occupancy by inserting or removing Idle sequences as necessary. This causes no corruption to the frames of data.
When the GMII is used as an internal interface, it is expected that the entire interface is synchronous to a single clock domain, and the TX elastic buffer should be discarded.