Information Technology Reference
In-Depth Information
The discard eligibility (DE) bit is set by the DTE to tell the Frame Relay network that a frame has lower
importance than other frames and should be discarded before other frames if the network becomes short
of resources. Thus, it represents a very simple priority mechanism. This bit is usually set only when the
network is congested.
LMI Message Format
The previous section described 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.
Figure18-3 The LMI Message Format
1
2
1
1
1
1
Variable
2
1
Field length,
in bytes
Unnumbered
information
indicator
Protocol
discriminator
LMI
DLCI
Call
reference
Message
type
Information
elements
Flag
FCS
Flag
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 (the
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.) The next byte is referred to as the
protocol discriminator
, which is set to a value that indicates LMI. The third mandatory byte (the
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.
