For success on your CCNA 640-802 & CCENT exams, you really need to understand how & 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 - & without LMI, you have no Frame Relay!
The Local
Management Interface (LMI) messages are sent between the DCE, typically the service provider, & 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 & DTE, the frame connection will drop. The LMI also shows
the PVC status to the router, reflected as either active or inactive.
The LMI types must match on the DTE & DCE for the PVC to be established. There’re
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, & Ultimate CCNP Study
Packages.
You can also visit his blog, which is updated many times daily with new Cisco certification articles, free tutorials, & daily CCNA / CCNP exam
questions!
For a FREE copy of his latest e-books, “How To Pass The CCNA” & “How To Pass The CCNP”, just visit the website! You can also get FREE
CCNA & CCNP exam questions every day!
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