- A keepalive mechanism, which verifies that
data is flowing
- A multicast mechanism, which provides the
network server with its local DLCI
- The multicast addressing, which gives DLCIs
global rather than local significance in Frame Relay networks
- A status mechanism, which provides an
ongoing status on the DLCIs known to the switch
Although the LMI is configurable,
beginning in Cisco IOS Release 11.2, the router tries to autosense which
LMI type the Frame Relay switch is using by sending one or more full
status requests to the Frame Relay switch. The Frame Relay switch
responds with one or more LMI types. The router configures itself
with the last LMI type received. Three types of LMIs are supported:
- cisco LMI - type, defined
jointly by Cisco, StrataCom, Northern Telecom, and Digital
Equipment Corporation, nicknamed “the gang of four”
- ansi Annex - D of the ANSI
standard T1.617
- q933a ITU-T Q.933 Annex A
An administrator setting up a
connection to a Frame Relay network must choose the appropriate LMI
from the three supported types to ensure proper Frame Relay
operation.
Frame Relay Extensions
In addition to the basic Frame Relay protocol
functions for transferring data, the "gang of four"
promoted the following extensions:
Virtual circuit status messages (common)-These
messages provide communication and synchronization between the
network and the user device. They periodically report the existence
of new PVCs and the deletion of already existing PVCs, and generally
provide information about PVC integrity. VC status messages prevent
the sending of data into black holes, that is, over PVCs that no
longer exist.
Multicasting (optional)-Multicasting
allows a sender to transmit a single frame, but have it delivered by
the network to multiple recipients. Thus, multicasting supports the
efficient conveyance of routing protocol messages and
address-resolution procedures that typically must be sent to many
destinations simultaneously.
Global addressing (optional) -Global
addressing gives connection identifiers global rather than local
significance, which allows them to be used to identify a specific
interface to the Frame Relay network. Global addressing makes the
Frame Relay network resemble a local-area network (LAN) in terms of
addressing. Therefore, address resolution protocols perform over
Frame Relay exactly as they do over a LAN.
Simple flow control (optional)-This
provides for an XON/XOFF flow-control mechanism that applies to the
entire Frame Relay interface. It is intended for those devices whose
higher layers cannot use the congestion notification bits and that
need some level of flow control.
When an inverse ARP request is made,
the router updates its map table with three possible LMI connection
states, as follows:
- Active state - indicates that the
connection is active and that routers can exchange data
- Inactive state - indicates that the
local connection to the Frame Relay switch is working, but the
remote router connection to the Frame Relay switch is not
working
- Deleted state - indicates that no
LMI is being received from the Frame Relay switch, or that there
is no service between the CPE router and Frame Relay switch