Compatible Systems 5.4 - Manual (Page 298)

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.