Enterasys Networks CSX7000 Manuel d'utilisateur télécharger pdf (Page 261)

Central Site Remote Access Switch 261

ONFIGURING

LTERNATE

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.

HE LOCAL MANAGEMENT INTERFACE OVERVIEW

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.

ATA RATE CONTROL OVERVIEW

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

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Enterasys Networks CSX7000 Manuel d'utilisateur Page 261

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