Various methods and systems for detecting whether a network device supports a protocol, which defines a supplemental header, are disclosed. One method involves detecting a value within a preamble generated by a sending device and verifying that a header format of a header, also generated by the sending device, conforms to a header format definition of a protocol. The header format definition of the protocol defines a supplemental header. The preamble can be an Ethernet preamble. In one embodiment, the preamble is a converged data link (CDL) preamble or other type of preamble that is used to convey operation, administration and management (OAM) information.