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Frame Relay LMI And LMI Autosense


For success on your CCNA 640-802 and CCENT exams, you need to understand how and why Frame Relay LMI messages operate. You need to know this for real-world networking as well, since Frame Relay is prevalent in today's networks - and without LMI, you have no Frame Relay!

The Local Management Interface (LMI) messages are sent between the DCE, typically the service provider, and the DTE, the Cisco router. LMI Status messages serve as keepalives for the frame connection. If keepalives are not continually received by both the DCE and DTE, the frame connection will drop. The LMI also indicates the PVC status to the router, reflected as either active or inactive.

The LMI types must match on the DTE and DCE for the PVC to be established. There are three types of LMI:
  • Cisco (the default)
  • ansi
  • q933a
The LMI type can be changed with the frame lmi-type command. Before doing anything with the frame relay commands, though, we have to enable frame relay on the interface with the encapsulation frame-relay command. Remember, the default encapsulation type on a Cisco Serial interface is HDLC.

R1(config)#interface serial0
R1(config-if)#encapsulation ?
  atm-dxi      ATM-DXI encapsulation
  frame-relay  Frame Relay networks
  hdlc         Serial HDLC synchronous
  lapb         LAPB (X.25 Level 2)
  ppp          Point-to-Point protocol
  smds         Switched Megabit Data Service (SMDS)
  x25          X.25

R1(config-if)#encapsulation frame-relay
R1(config-if)#frame-relay lmi-type ?
  cisco
  ansi
  q933a


We can hardcode the LMI type as shown in that example, and there's another way to get the LMI to match with the remote DCE. LMI Autosense has the router send out an LMI Status message for all three LMI types.

The router then waits for a response for one of those LMI types from the DCE. When the router sees the response to its LMI Autosense messages, the router will then send only the same LMI type it received from the DCE.

On rare occasions - such as your CCNA exam, perhaps :) - the Frame LMI may not match. We'll take a look at such a scenario in the next installment of my exclusive Cisco CCENT / CCNA 640-802 certification exam tutorial series!

About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!





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