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258 Chapter 9: Frame Relay Connection Controlling Traffic Flow

DTEs are generally considered to be terminating equipment for a specific network and are
located at the customer premises. The customer typically owns the DTEs. Examples of DTE
devices are terminals, personal computers, routers, and LAN switches.

DCEs are carrier-owned internetworking devices. DCE equipment provides clocking and
switching services in a network; they are the devices that actually transmit data through the
WAN. In most cases, the devices are packet switches.

Figure 9-2 depicts the roles of Frame Relay devices as discussed to this point.

Figure 9-2

Frame Relay Device Roles

Frame Relay LMI

Local Management Interface (LMI) is the means by which Frame Relay edge devices maintain
keepalive messages. The Frame Relay switch is responsible for maintaining the status of the CPE
device(s) to which it is attached. LMI is the communication by which the switch monitors status.

LMI implements a keepalive mechanism that verifies connectivity between DCE and DTE and
the fact that data can flow. A LMI multicast capability, in conjunction with an LMI multicast
addressing mechanism, enables attached devices to learn local DLCIs as well as provide global,
rather than local, significance to those DLCIs. Finally, LMI provides a status indicator that is
constantly exchanged between router and switch.

The LMI setting is configurable; however, the router still tries to figure out the type on its own.
(This statement is true as of IOS 11.2. Prior to 11.2 IOS, the LMI setting was manually
configured based on telco requirements.)

There are three supported types of LMI (can you guess the default?):

Cisco--Created by the "gang of four," this type was defined by a joint effort between
Cisco, StrataCom, Nortel, and DEC. Cisco LMI uses DLCI 1023.

ANSI--This type is also known as Annex D. (Actually, the full version of the name is
ANSI standard T1.617 Annex D.) ANSI LMI uses DLCI 0.

Q933a--This type, defined by the ITU, is known as Annex A. (Its full name is actually
ITU-T Q.933 Annex A.)

DCE

CSU/DSU

DCE

DTE

Frame Relay

switch cloud