6.2 LMI: Cisco's Implementation of Frame Relay
6.2.2 The fields of the LMI frame format
The Frame Relay specification also includes the LMI procedures. LMI messages are sent in frames distinguished by an LMI-specific DLCI (defined in the consortium specification as DLCI = 1023). The LMI frame format is shown in the Figure.

After the flag and LMI DLCI fields, the LMI frame contains 4 mandatory bytes. The first of the mandatory bytes (unnumbered information indicator) has the same format as the LAPB unnumbered information (UI) frame indicator, with the poll/final bit set to zero. The next byte is referred to as the protocol discriminator, which is set to a value that indicates LMI. The third mandatory byte (call reference) is always filled with zeros.

The final mandatory byte is the message type field. Two message types have been defined: status messages and status-enquiry messages . Status messages respond to status-enquiry messages. Examples of these messages  are (1) keepalives (messages sent through a connection to ensure that both sides will continue to regard the connection as active) and (2) a status message of an individual report on each DLCI defined for the link. These common LMI features are expected to be a part of every implementation that conforms to the Frame Relay specification. 

Together, status and status-enquiry messages help verify the integrity of logical and physical links. This information is critical in a routing environment because routing protocols make decisions based on link integrity.

Next is an information element (IE) field of a variable number of bytes. Following the message type field is some number of IEs. Each IE consists of a 1-byte IE identifier, an IE length field, and 1 or more bytes containing actual data.