Figure 1515 LMI Frame Format

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Table 15-8 presents a description of each LMI field.

Table 15-8. LMI Frame Format Field Description

Field

Description

Flag

Delimits the start and end of the LMI frame.

LMI DLCI

Identifies the frame as an LMI frame rather than a Frame Relay data frame. The DLCI value is dependent on the LMI specification used; LMI (original) uses DLCI 1023, LMI (Annex A) and LMI (Annex D) use DLCI 0.

Unnumbered

Information

Indicator

Sets the poll/final bit to zero (0).

Protocol Discriminator

Always contains a value indicating that the frame is an LMI frame.

Call Reference

Always contains zeros. This field is currently not used for any purpose.

Message Type

Labels the frame as one of the following message types:

• Status-inquiry message— Allows a user device to inquire about the status of the network.

• Status message— Responds to status-inquiry messages. Status messages include keepalive and PVC status messages.

Information Elements

Contains a variable number of individual information elements (IEs). IEs consist of the following fields:

• IE Identifier— Uniquely identifies the IE.

• Data— Consists of one or more bytes containing encapsulated upper-layer data.

Frame Check Sequence (FCS)

Ensures the integrity of transmitted data.

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