5.1 Frame Relay Concepts  
  5.1.6 Frame Relay LMI  
Frame Relay was designed to provide packet-switched data transfer with minimal end-to-end delays. Anything that might contribute to delays was omitted. When vendors came to implement Frame Relay as a separate technology rather than as one component of ISDN, they decided that there was a need for DTEs to dynamically acquire information about the status of the network. This feature was omitted in the original design. The extensions for this status transfer are called the Local Management Interface (LMI).

The 10-bit DLCI field allows VC identifiers 0 through 1023. The LMI extensions reserve some of these identifiers. This reduces the number of permitted VCs. LMI messages are exchanged between the DTE and DCE using these reserved DLCIs.

The LMI extensions include the following:

  • The heartbeat mechanism, which verifies that a VC is operational
  • The multicast mechanism
  • The flow control
  • The ability to give DLCIs global significance
  • The VC status mechanism

There are several LMI types, each of which is incompatible with the others. The LMI type configured on the router must match the type used by the service provider. Three types of LMIs are supported by Cisco routers:

  • Cisco – The original LMI extensions
  • Ansi – Corresponding to the ANSI standard T1.617 Annex D
  • q933a – Corresponding to the ITU standard Q933 Annex A

LMI messages are carried in a variant of LAPF frames. This variant includes four extra fields in the header so that they will be compatible with the LAPD frames used in ISDN. The address field carries one of the reserved DLCIs. Following this are the control, protocol discriminator, and call reference fields that do not change. The fourth field indicates the LMI message type.

There are one or more information elements (IE) that follow the header. Each IE consists of the following:

  • A one byte IE identifier
  • An IE length field
  • One or more bytes containing actual data that typically includes the status of a DLCI

Status 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.

 

Interactive Media Activity

Drag and Drop: LMI Message Format

When the student has completed this activity, the student will be able to correctly order the fields in a LMI message frame.

   
 

Web Links

Frame Relay

http://www.tele.sunyit.edu/ internetworking/ 55149.htm