LMI protocol
Frame relay uses the LMI protocol to set up virtual circuits and maintain their status between DTE and DCE.
The system supports the following LMI standards:
ITU-T Q.933 Annex A
ANSI T1.617 Annex D
Nonstandard LMI (compatible with other vendors)
To communicate, the DTE and the DCE must use the same type of LMI.
LMI uses the status inquiry message and the status messages to maintain the link status and PVC status, for example, to advertise new PVCs, detect deleted PVCs, monitor PVC status changes, and verify link integrity. For these purposes, the DTE sends status inquiry messages regularly to the DCE to request for the availability of individual PVCs. On receiving a status inquiry, the DCE responds with a status message that describes the status of each virtual circuit on the physical link.
For a DTE, the status of a PVC is determined by the DCE; as for DCE, by the frame relay network.
Table 2 lists the major parameters ITU-T Q.933 Annex A uses for message exchange. You can configure these parameters to optimize device performance.
Table 2: Parameter description for frame relay protocol
Device role | Timer/counter | Value range | Default value | Description |
|---|---|---|---|---|
DTE | Full status polling counter (N391) | 1 to 255 | 6 | Sets the frequency with which status inquires expect a full status report. Suppose the N391 is set to 5. Every fifth status inquiry sent by the DTE will ask for a full status report. The intermediate inquiries ask for a keepalive exchange only. |
Error threshold counter (N392) | 1 to 10 | 3 | Sets the number of errors required for LMI to declare a link dead, within the event count specified by N393. | |
Monitored events counter (N393) | 1 to 10 | 4 | Sets the monitored event count. If the number of errors within the N393 status inquiries reaches N392, the DTE considers that the error threshold is reached. | |
Keepalive (link integrity verification polling) timer (T391) | 0 to 32767 0 means LMI disabled. | 10 | Sets the interval (in seconds) at which the DTE sends a status inquiry message. An error is recorded if no reply is received before the timer expires. | |
DCE | Error threshold counter (N392) | 1 to 10 | 3 | Sets the number of errors required for LMI to declare a link dead, within the event count specified by N393. |
Monitored events count (N393) | 1 to 10 | 4 | Sets the monitoring interval for error counts. | |
Keepalive (polling verification) timer (T392) | 5 to 30 | 15 | Sets the interval (in seconds) for receiving a status inquiry message. If no status inquiry message is received before the timer expires, an error is recorded. |