System information

Frame Relay Technology Basics
Book Title
18-400
LMI Message Format
The previous section describes the basic Frame Relay protocol format for carrying user data frames.
The consortium Frame Relay specification also includes the LMI procedures. LMI messages are
sent in frames distinguished by an LMI-specific DLCI (defined in the consortium specification as
DLCI = 1023). The LMI message format is shown in Figure 18–3.
Figure 18-3 The LMI Message Format
In LMI messages, the basic protocol header is the same as in normal data frames. The actual LMI
message begins with 4 mandatory bytes, followed by a variable number of information elements
(IEs). The format and encoding of LMI messages is based on the ANSI T1S1 standard.
The first of the mandatory bytes (unnumbered information indicator) has the same format as the
Link Access Procedure, Balanced (LAPB) unnumbered information (UI) frame indicator with the
poll/final bit set to 0. (For more information about LAPB, see Chapter 19, “Troubleshooting X.25
Connections.”) The next byte is referred to as the protocol discriminator, which is set to a value that
indicates LMI. The third mandatory byte (call reference) is always filled with zeros.
The final mandatory byte is the Message type field. Two message types have been defined.
Status-enquiry messages allow the user device to inquire about network status. Status messages
respond to status-enquiry messages. Keepalives (messages sent through a connection to ensure that
both sides will continue to regard the connection as active) and PVC status messages are examples
of these messages and are the common LMI features that are expected to be a part of every
implementation that conforms to the consortium specification.
Together, status and status-enquiry messages help verify the integrity of logical and physical links.
This information is critical in a routing environment because routing algorithms make decisions
based on link integrity.
Following the message type field is some number of IEs. Each IE consists of a single-byte IE
identifier, an IE length field, and 1 or more bytes containing actual data.
Global Addressing
In addition to the common LMI features, several optional LMI extensions are extremely useful in an
internetworking environment. The first important optional LMI extension is global addressing. As
noted previously, the basic (nonextended) Frame Relay specification supports only values of the
DLCI field that identify PVCs with local significance. In this case, no addresses identify network
interfaces or nodes attached to these interfaces. Because these addresses do not exist, they cannot be
discovered by traditional address resolution and discovery techniques. This means that with normal
Frame Relay addressing, static maps must be created to tell routers which DLCIs to use to find a
remote device and its associated internetwork address.
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.
Field length,
in bytes
111 1 122Variable
Flag
Message
type
Information
elements
LMI
DLCI
FCS
Unnumbered
information
indicator
Protocol
discriminator
Flag
Call
reference
1