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Cisco CCNA 640-802 - CCENT Certification Exam Training - Frame Relay LMI And LMI Autosense

Published Monday, July 21, 2008 by bloggerblognet22596 | E-mail this post

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!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free Cisco CCNA 640-802 certification, CCENT tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.

You can also visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions!

For a FREE copy of his latest e-books, "How To Pass The CCNA" and "How To Pass The CCNP", just visit the website! You can also get FREE CCNA and CCNP exam questions every day!

Take the CCNA Mastermind Webinar Boot Camp with The Bryant Advantage!

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