Information Technology Reference
In-Depth Information
The global addressing extension permits node identifiers. With this extension, the values inserted in the
DLCI field of a frame are globally significant addresses of individual end-user devices (for example,
routers). This is implemented as shown in Figure 18-4.
Figure18-4 Global Addressing Exchange
San Jose
Pittsburgh
DLCI = 12
DLCI = 13
Switch
Switch
Switch
Switch
Switch
WAN
Router
DLCI = 14
DLCI = 15
Los Angeles
Atlanta
In Figure 18-4, note that each interface has its own identifier. Suppose that Pittsburgh must send a frame
to San Jose. The identifier for San Jose is 12, so Pittsburgh places the value 12 in the DLCI field and
sends the frame into the Frame Relay network. At the exit point, the DLCI field contents are changed by
the network to 13 to reflect the source node of the frame. Each router interface has a distinct value as its
node identifier, so individual devices can be distinguished. This permits adaptive routing in complex
environments.
Global addressing provides significant benefits in a large, complex internetwork. The Frame Relay
network now appears to the routers on its periphery like any LAN. No changes to higher-layer protocols
are needed to take full advantage of their capabilities.
Multicasting
Multicasting is another valuable optional LMI feature. Multicast groups are designated by a series of
four reserved DLCI values (1019 to 1022). Frames sent by a device using one of these reserved DLCIs
are replicated by the network and sent to all exit points in the designated set. The multicasting extension
also defines LMI messages that notify user devices of the addition, deletion, and presence of multicast
groups.
In networks that take advantage of dynamic routing, routing information must be exchanged among
many routers. Routing messages can be sent efficiently by using frames with a multicast DLCI. This
allows messages to be sent to specific groups of routers.
Frame Relay Configuration
When implementing Frame Relay—and, specifically, LMI—in an internetwork, the LMI type must be
consistent across all points between the end devices. The three LMI types that can be used are ANSI,
Cisco, and Q.933A. The current Internetworking Operating System (IOS) on Cisco routers autosenses
the LMI type from the frame switch to which it is attached.
