|  | 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.