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.
|