LMI protocol
Frame Relay uses the LMI protocol to manage PVCs, including the following operations:
Notify the addition of a PVC.
Detect the deletion of a PVC.
Monitor PVC status changes.
Verify link integrity.
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 properly, the DTE and the DCE must use the same type of LMI.
LMI messages
LMI messages include the following types:
Status enquiry message—A DTE sends status enquiry messages regularly to a DCE to request the status of individual PVCs or verify the link integrity.
Status message—When a DCE receiving a status enquiry message, the DCE responds with a status message. The status message is used to transmit the PVC status or verify the link integrity.
Status enquiry messages and status messages include the following types:
Full status—Verifies the link integrity and transmits the PVC status.
Link integrity verification (LIV)—Verifies the link integrity.
LMI negotiation parameters
Table 3 lists the parameters ITU-T Q.933 Annex A uses for message exchange. You can configure these parameters to optimize device performance.
Table 3: LMI negotiation parameters
Device role | Timer/counter | Value range | Default value | Description |
|---|---|---|---|---|
DTE | Full status polling counter (N391) | 1 to 255 | 6 | Sets the ratio of link integrity request messages sent to full status enquiry messages sent. The ratio is (N391-1):1. |
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 enquiry messages reaches N392, a 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 a DTE sends a status enquiry message. An error is recorded if the DTE has not received any replies when 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 monitored event count. If the number of errors within the N393 status enquiry messages reaches N392, a DCE considers that the error threshold is reached. | |
Keepalive (polling verification) timer (T392) | 5 to 30 | 15 | Sets the interval (in seconds) for receiving a status enquiry message. If a DCE has not received any status enquiry messages when the timer expires, an error is recorded. |
How LMI works
LMI works in the following process:
When a DTE physically goes up, it sends a full status enquiry message to a DCE to request the virtual circuit status. At the same time, the T391 timer and the V391 counter start. The T391 timer specifies the polling interval at which the DTE sends status enquiry messages.
When V391<N391, the DTE sends a link integrity verification message and requests only the link integrity.
When V391=N391, the following events occur:
V391 is reset to 0.
The DTE sends a full status enquiry message to request not only the link integrity but also the status of all PVCs.
When the DCE receives a request, the DCE sends a status message that carries the status requested by the DTE. At the same time, the DCE starts the T392 timer and waits for the next status enquiry message. If the DCE has not received status enquiry messages when the T392 timer expires, the DCE records the error and increases the error count by one. If the number of errors exceeds N392 among N393 events, the DCE considers the physical link and all virtual circuits unavailable and will not use them to forward packets.
When the DTE receives a reply, the DTE updates the link status and PVC status. If the DTE has not received status messages when the T391 timer expires, the DTE records the error and increases the error count by one. If the number of errors exceeds N392 among N393 events, the DTE considers that the physical link and all virtual circuits unavailable and will not use them to forward packets.