In last week's newsletter, I started writing about the Frame Relay WAN protocol.
I covered the basics of Frame, including the types of Frame circuits and basic
congestion control mechanisms. This week, I will take the Frame Relay topic into
the 90's, with a discussion on extensions to the basic Frame Relay specification
as well as the format of Frame Relay frame.
In 1990, Cisco and several
partners developed the Local Management Interfaces (LMI) enhancements to the
Frame protocol. LMI adds several important management functions to the Frame
specifications. There are three types of LMI configurable on Cisco devices:
Cisco, ANSI, and q933a (default is Cisco). LMI uses reserved DLCI 1023 (cisco
LMI) or DLCI 0 (ANSI and ITU).
When a DTE device connects to a Frame
switch, LMI advertises which DLCIs available to the DTE are active. LMI also
changes a DLCI from a local (to a PVC) identifier to a global (entire network)
one. This allows DTE device to be identified by DLCI, making a Frame network
more like a regular LAN.
LMI monitors and reports on the status of PVCs.
Any changes to the status of a PVC can be forwarded to other devices on the WAN.
LMI multicast groups can be specified to multicast PVC and DLCI status updates.
LMI uses reserved DLCI 1023 (cisco LMI) or DLCI 0 (ANSI and ITU).
A
standard Frame Relay frame is composed of four main parts. There are one-byte
flag fields that mark the beginning and end of each frame. There is a 2-byte
address field that contains the DLCI and congestion control information. The
variable-length (up to 16000 bytes) data or payload field follows the address
field. The data field is followed by a 2-byte frame check sequence (FCS) that is
used to ensure the integrity of the frame.
An LMI frame is different in
several ways. There is still a flag at the beginning and the end of the frame.
Following the start of frame flag is the 2-byte LMI DLCI that indicates that the
frame is LMI instead of basic Frame Relay. After the LMI DLCI frame is the
1-byte Unnumbered Information Indicator that sets the poll/final bit to zero.
This is followed by a 1-byte Protocol Discriminator field that contains
a value that indicates that the frame is an LMI frame. After the Protocol field
is the 1-byte Call Reference field. The Call Reference field is always all zeros
and is not used.
A 1-byte Message Type frame follows the Call Reference
Frame. The Message Type frame is either a status-inquiry (inquires about network
status) or a status message (Response to status-inquiry message-keepalives and
PVC status). The next field is a Information Elements field that can contain
multiple information elements in the format IE identifier, IE Length, and data
(one or more bytes of encapsulated data). Right before the end of frame flag is
the frame check sequence.
This week, I wrote about the LMI enhancements
to the Frame Relay standards and the format of a Frame Relay. The links below
provide more information, including graphics of the frame formats. Next week,
we'll be going into the lab to learn how to configure Frame on Cisco devices.
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