Enterasys Networks CSX6000 - Manual (Page 261)

Central Site Remote Access Switch 261
C ONFIGURING A LTERNATE A CCESSES
Frame Relay Accesses
configured in the device table. It will find the PVC and the line protocol that corresponds to the
PVC name and change its PVC name to match the corresponding device name.
Notes: Connection Services Manager (CSM) is currently the only off-node device database
supported by the CyberSWITCH for Frame Relay.
The management of Frame Relay permanent virtual circuits requires the use of some form
of security. Therefore, systems with frame relay remote devices cannot select No Security
for the security level.
Frame Relay uses ISDN to provide backup redundancy for failed Frame Relay links. In the event
that a Frame Relay link fails, an ISDN call is brought up and all traffic that was to be forwarded on
the Frame Relay link is forwarded over the ISDN call. Once the Frame Relay link comes back up
the ISDN call will be taken down and transmission of data will resume over the Frame Relay link.
When using FR_IETF data encapsulation, LMI is used to determine the status of failed Frame Relay
virtual circuits. When using PPP data encapsulation, Link Failure Detection can be enabled for the
Frame Relay access. Link Failure Detection is only supported across PPP permanent virtual circuits.
Some Frame Relay networks have a per packet charge, therefore, the administrator should be
cautious when enabling this feature.
The three main operational components of a frame relay access are the Local Management Interface
(LMI), Data Rate Control, and Congestion Control. The following three sections provide an
overview of the role that each of these components plays in the function of frame relay access.
T HE L OCAL M ANAGEMENT I NTERFACE O VERVIEW
Besides the steady state data transfer portion of the frame relay module, the standards have
provided for a frame relay management function, known as the Local Management Interface (LMI).
The purpose of this interface is to provide a controlled means of verifying both link integrity and
the network status of all PVCs configured on the frame relay access defined by a given physical
link.
The elements of this management interface are the STATUS and STATUS_ENQUIRY messages.
The STATUS_ENQUIRY messages are sent out by the user equipment at regular intervals. The
interval at which these status messages are sent, the polling interval, is a configurable value. The
network will respond to these STATUS_ENQUIRY messages with its own STATUS message
containing a link integrity verification information element. The user equipment will request via
the STATUS_ENQUIRY either a Link Integrity Verification STATUS message from the network or
a full report STATUS message. The link integrity verification STATUS message simply functions as
a signal to verify that the link is still operable. This full report signals the user equipment when a
PVC is no longer usable, and also when a previously non-active PVC has become available.
D ATA R ATE C ONTROL O VERVIEW
To handle congestion within the network and at the endpoints, the frame relay protocol provides
certain congestion control features.
A frame relay PVC has two transmission rates associated with it: the Committed Information Rate
(CIR) and an Excess Information Rate (EIR). The committed information rate is the bandwidth
requested for a PVC at service subscription time. It is essentially the guaranteed transmission rate