Local Management Interface (LMI) is a signaling standard used between your router and the first Frame Relay switch it’s connected to. It allows for passing information about the operation and status of the virtual circuit between the provider’s network and the DTE (your router).
It communicates information about the following:
Keepalives These verify that data is flowing.
Multicasting This is an optional extension of the LMI specification that allows, for example, the efficient distribution of routing information and ARP requests over a Frame Relay network. Multicasting uses the reserved DLCIs from 1019 through 1022.
Global addressing This provides global significance to DLCIs, allowing the Frame Relay cloud to work exactly like a LAN.
Status of virtual circuits This provides DLCI status. The status inquiries and messages are used as keepalives when there is no regular LMI traffic to send.
But remember, LMI is not communication between your routers; it’s communication between your router and the nearest Frame Relay switch. So it’s entirely possible that the router on one end of a PVC is actively receiving LMI while the router on the other end of the PVC is not. And of course, PVCs won’t work with one end down. (I say this to clarify the local nature of LMI communications.)
There are three different types of LMI message formats: Cisco, ANSI, and Q.933A. The different kinds in use depend on both the type and configuration of the telco’s switching gear, so it’s imperative that you configure your router for the correct format, which should be provided by the telco.
Note: Beginning with IOS version 11.2, the LMI type is autosensed. This enables the interface to determine the LMI type supported by the switch. If you’re not going to use the autosense feature, you’ll need to check with your Frame Relay provider to find out which type to use instead.
On Cisco equipment, the default type is, surprise, Cisco, but you still might have to change to ANSI or Q.933A depending on what your service provider tells you.
The three different LMI types are shown in the following router output:
RouterA(config-if)#frame-relay lmi-type ?
cisco
ansi
q933a
As seen in the output, all three standard LMI signaling formats are supported. Here’s a description of each:
Cisco LMI defined by the Gang of Four (default). The Local Management Interface (LMI) was developed in 1990 by Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment
Corporation and became known as the Gang-of-Four LMI, or Cisco LMI.
ANSI Annex D included with ANSI standard T1.617.
ITU-T (Q.933A) Annex A included in the ITU-T standard and defined by using the Q.933a command keyword.
Routers receive LMI information from the service provider’s Frame Relay switch on a frame encapsulated interface and update the virtual circuit status to one of three different states:
Active state Everything is up, and routers can exchange information.
Inactive state The router’s interface is up and working with a connection to the switching office, but the remote router isn’t up.
Deleted state No LMI information is being received on the interface from the switch, which could be due to a mapping problem or a line failure.
It communicates information about the following:
Keepalives These verify that data is flowing.
Multicasting This is an optional extension of the LMI specification that allows, for example, the efficient distribution of routing information and ARP requests over a Frame Relay network. Multicasting uses the reserved DLCIs from 1019 through 1022.
Global addressing This provides global significance to DLCIs, allowing the Frame Relay cloud to work exactly like a LAN.
Status of virtual circuits This provides DLCI status. The status inquiries and messages are used as keepalives when there is no regular LMI traffic to send.
But remember, LMI is not communication between your routers; it’s communication between your router and the nearest Frame Relay switch. So it’s entirely possible that the router on one end of a PVC is actively receiving LMI while the router on the other end of the PVC is not. And of course, PVCs won’t work with one end down. (I say this to clarify the local nature of LMI communications.)
There are three different types of LMI message formats: Cisco, ANSI, and Q.933A. The different kinds in use depend on both the type and configuration of the telco’s switching gear, so it’s imperative that you configure your router for the correct format, which should be provided by the telco.
Note: Beginning with IOS version 11.2, the LMI type is autosensed. This enables the interface to determine the LMI type supported by the switch. If you’re not going to use the autosense feature, you’ll need to check with your Frame Relay provider to find out which type to use instead.
On Cisco equipment, the default type is, surprise, Cisco, but you still might have to change to ANSI or Q.933A depending on what your service provider tells you.
The three different LMI types are shown in the following router output:
RouterA(config-if)#frame-relay lmi-type ?
cisco
ansi
q933a
As seen in the output, all three standard LMI signaling formats are supported. Here’s a description of each:
Cisco LMI defined by the Gang of Four (default). The Local Management Interface (LMI) was developed in 1990 by Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment
Corporation and became known as the Gang-of-Four LMI, or Cisco LMI.
ANSI Annex D included with ANSI standard T1.617.
ITU-T (Q.933A) Annex A included in the ITU-T standard and defined by using the Q.933a command keyword.
Routers receive LMI information from the service provider’s Frame Relay switch on a frame encapsulated interface and update the virtual circuit status to one of three different states:
Active state Everything is up, and routers can exchange information.
Inactive state The router’s interface is up and working with a connection to the switching office, but the remote router isn’t up.
Deleted state No LMI information is being received on the interface from the switch, which could be due to a mapping problem or a line failure.
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