H3C Technologies H3C SR8800 - Manual (Page 61)

53
Virtual circuit
Virtual circuits fall into permanent virtual circuits (PVCs) and switched virtual circuits (SVCs), depending
on how they are set up. Virtual circuits configured manually are called “PVCs”, and those created by
protocol negotiation are called “SVCs”, which are automatically created and deleted by the frame relay
protocol. The most frequently used in frame relay is the PVC, which is a manually configured virtual
circuit.
The PVC status on DTE is completely determined by DCE, and the network determines the PVC status on
DCE. If two routers are directly connected, the equipment administrator sets the virtual circuit status of
DCE.
Data link connection identifier
A data link connection identifier (DLCI) is a unique number assigned to a virtual circuit endpoint in a
frame relay network for the addressing purpose.
A DLCI uniquely identifies a particular virtual circuit on a physical link and has local significance only to
that link. A DLCI can be used on different physical ports to address different virtual circuits and a virtual
circuit between two DTE devices may be addressed with different DLCIs at the two ends.
Because the virtual circuits in a frame relay network are connection oriented, each DLCI on a physical
port is destined for a distinct remote device. DLCIs can be regarded the frame relay addresses of remote
devices.
The maximum number of PVCs that can be created on a frame relay interface is 1024. The user
configurable DLCIs for the PVCs are in the range 16 to 1007. Other DLCIs are reserved for special
purposes. For example, DLCI 0 and DLCI 1023 are reserved for the LMI protocol to transfer control
messages.
LMI protocol
Frame relay uses the Local Management Interface (LMI) protocol to set up virtual circuits and maintain
their status between DTE and DCE.
The system supports the following LMI standards:
• ITU-T Q.933 Annex A
• ANSI T1.617 Annex D
• Nonstandard LMI (compatible with other vendors)
To communicate properly, the DTE and the DCE must use the same type of LMI.
LMI uses the status inquiry message and the status messages to maintain the link status and PVC status,
for example, to advertise new PVCs, detect deleted PVCs, monitor PVC status changes, and verify link
integrity. For these purposes, the DTE sends status inquiry messages regularly to the DCE to request for
the availability of individual PVCs. On receiving a status inquiry, the DCE responds with a status
message that describes the status of each virtual circuit on the physical link.
For a DTE, the status of a PVC is determined by the DCE; as for DCE, by the frame relay network.
Table 2 lists the major parameters ITU-T Q.933 Annex A uses for message exchange. You can configure
these parameters to optimize device performance.