Frame Relay Polling, Error Handling, and Specification Enhancements - Page 4

By Cisco Press | Posted Jan 25, 2002
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LMI Types
Three types of LMI are found in Frame Relay network implementations:

  • ANSI T1.617 (Annex D)Maximum number of connections (PVCs) supported is limited to 976. LMI type ANSI T1.627 (Annex D) uses DLCI 0 to carry local (link) management information.
  • ITU-T Q.933 (Annex A)Like LMI type Annex D, the maximum number of connections (PVCs) supported is limited to 976. LMI type ITU-T Q.933 (Annex A) also uses DLCI 0 to carry local (link) management information.
  • LMI (Original)Maximum number of connections (PVCs) supported is limited to 992. LMI type LMI uses DLCI 1023 to carry local (link) management information.


NOTE:   LMI Type LMI (Original) is annotated as LMI type Cisco within the Cisco IOS.


NOTE:   The frame MTU setting impacts LMI messages. If PVCs appear to be bouncing, (that is, repeated up/down indications), it might be because of the MTU size of the Frame Relay frame. If the MTU size is too small, not all PVC status messages will be communicated between the service provider edge and the Frame Relay access router. If this condition is suspected, the next step is to contact the network service provider to troubleshoot.

LMI Frame Format
Figure 15-15 illustrates the LMI frame format to which Frame Relay LMI frames must conform, as deemed by the LMI specification.

Figure 15-15: LMI Frame Format
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(Click image for larger view in a new window)

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 IdentifierUniquely identifies the IE.
  • IE LengthIndicates the length of the IE.
  • DataConsists of one or more bytes containing encapsulated upper-layer data.
Frame Check Sequence (FCS) Ensures the integrity of transmitted data.

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