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292 Appendices

Local & Global DLCIs

Local DLCI addressing means that DLCI numbers are only significant at one

end of a Frame Relay virtual circuit (VC). In other words, the same VC will

be identified by different DLCIs at each end. To accomplish this, a mapping

occurs across a VC. Frame Relay switches are required to translate the

“source” DLCI in a packet to the “destination” DLCI when forwarding the

packet.

Global DLCI addressing is a Local Management Interface (LMI) extension

that allows a DLCI number to have universal significance. A global DLCI

identifies the same VC at both ends. Global addressing simplifies address

administration but allows for only 1024 DLCIs in the entire network. The

switch is not required to translate the DLCI in a packet as it does with local

DLCIs.

v Note: The majority of Frame Relay connections use Local DLCI

addressing, where a DLCI number is only significant at one end of the PVC.

Local Management Interface (LMI)

The local management interface specifies communication between different

Frame Relay devices (i.e. frame relay switches, routers, access devices, etc.).

Over the years, three different local management interface specifications

have been developed for Frame Relay: “consortium” LMI (an early coopera-

tive effort by a group of frame relay vendors), CCITT Annex A, and ANSI

Annex D. The CCITT and ANSI specifications are formal outgrowths of the

consortium LMI specification.

The Annex D specification is the most widely used in the United States,

although consortium LMI is still in use by some carriers. The Annex A spec-

ification is primarily a European specification.

Encapsulation and Fragmentation

RFC 1490 describes an encapsulation method for carrying packets across a

Frame Relay network. All protocol packets are encapsulated within a Q.922

Annex A frame (a CCITT specification for data frames). Additionally, the

frames must contain information necessary to identify the protocol being

carried, allowing the receiver to properly process the incoming packet.

RFC 1490 also specifies a simple fragmentation procedure for carrying large

frames over a frame relay network with a smaller maximum frame size.

Network/Protocol Addressing and Virtual Interfaces

Routing between LANs across a Frame Relay network is similar to routing

across a point-to-point connection. A PVC on one router is directly connected

to a PVC on another router. The difference is that multiple PVCs can be

supported on the same physical interface of a router.

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Compatible-systems 5.4 Manuel d'utilisateur Page 298