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.
The Inverse ARP mechanism allows the
router to automatically build the Frame Relay map, as shown in the
Figure. The router
learns the DLCIs that are in use from the switch during the initial
LMI exchange. The router then sends an Inverse ARP request to each
DLCI for each protocol configured on the interface if the protocol is
supported. The return information from the Inverse ARP is then used to
build the Frame Relay map.
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