Frame Relay Polling, Error Handling, and Specification Enhancements - Page 3
Frame Relay LMI
The Frame Relay LMI is a set of Frame Relay specification enhancements. The original LMI was developed in 1990 by the Gang of Four (Cisco, DEC, Nortel, and StrataCom). LMI includes support for the following:
- Keepalive mechanismsVerify the flow of data
- Multicast mechanismsProvide the network server with local and multicast DLCI information
- Global addressingGive DLCIs global rather than local significance
- Status mechanismsProvide ongoing status reports on the switch-known DLCIs
Figure 15-14: LMI Status Message Endpoints
(Click image for larger view in a new window)
The original LMI supports a number of features, or enhancements, to the original Frame Relay protocol, for managing Frame Relay internetworks. The most notable Frame Relay LMI extensions include support for the following:
- Global addressing -- The LMI global addressing extension gives Frame Relay DLCI values a global, rather than local, significance. These global DLCI values become Frame Relay networking device addresses that are unique in the Frame Relay WAN.
NOTE: As discussed earlier in this chapter, global addressing has an inherent limitation in that no more than 992 DLCIs (1024 DLCIs less the 32 reserved DLCIs) can be used. In a Frame Relay network of more than 992 sites, global addressing will not work. Apart from global addressing of DLCIs, the LMI status message presents an inherent limitation on the number of DLCIs that can be supported by an interface. Cisco has published a brief detailing these limitations at http://www.cisco.com/warp/public/125/lmidlci.html.
- Virtual circuit status messagesProvide communication and synchronization between Frame Relay network access devices (FRADs) and the network provider devices (switches). These messages report (in a regular interval) the status of PVCs, which prevents data from being pointed to a PVC that does not exist.
- MulticastingSupports the assignment management of multicast groups. Multicasting preserves bandwidth by enabling routing updates and address-resolution (such as ARP, RARP) messages to be sent only to specific groups of routers.
LMI VC status messages provide communication and synchronization between Frame Relay DTE and DCE devices. These messages are used to periodically report on the status of PVCs, which prevents data from being sent into black holes (over PVCs that no longer exist).
