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Frame Relay

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Frame Relay uses DLCIs the same way that X.25 uses X.121 addresses,

and every DLCI number can be given either global or local meaning every-
where within the Frame Relay network.

Sometimes a provider can give a site a DLCI that is advertised to all

remote sites as the same PVC. This PVC is said to have a global significance.
For example, a corporate office might have a DLCI of 20. All remote sites
would know that the corporate office is DLCI 20 and use this PVC to com-
municate to the corporate office. However, the customary implementation is
to give each DLCI local meaning. What does this mean? It means that DLCI
numbers do not necessarily need to be unique. Two DLCI numbers can be
the same on different sides of a link because Frame Relay maps a local DLCI
number to a virtual circuit on each interface of the switch. Basically, this
means that a DLCI is only locally significant to the physical serial interface.
Each remote office can have its own DLCI number and communicate with
the corporate office using unique DLCI numbers, which is called the local
significant DLCI number.

DLCI numbers, used to identify a PVC, are typically assigned by the

provider and start at 16. Configuring a DLCI number to be applied to an
interface is shown below:

RouterA(config-if)#frame-relay interface-dlci ?

<16-1007> Define a DLCI as part of the current

subinterface

RouterA(config-if)#frame-relay interface-dlci 16

Local Management Interface (LMI)

The Local Management Interface (LMI) was developed in 1990 by Cisco
Systems, StrataCom, Northern Telecom, and Digital Equipment Corpora-
tion and became known as the Gang-of-Four LMI or Cisco LMI. This gang
took the basic Frame Relay protocol from the CCITT and added extensions
onto the protocol features that allow internetworking devices to communi-
cate easily with a Frame Relay network.

The LMI is a signaling standard between a CPE device (router) and a frame

switch. The LMI is responsible for managing and maintaining status between
these devices. LMI messages provide information about the following:

Keepalives Verify data is flowing.

Multicasting Provides a local DLCI PVC.

Multicast addressing Provides global significance.

Status of virtual circuits Provides DLCI status.

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