Switched Virtual Circuit (SVC)
An SVC is established on per-call basis. That is an SVC is sporadic and established on-demand for duration of the call. The simplest example is of a telephone call, when a call is placed, the circuit is reserved for the duration of the call and resources are released as the call is terminated. Another example is of Dial-on-Demand Routing (DDR), where a circuit is established and maintained between two nodes if an interesting traffic exists. As soon as the system detects that there is no data to be transferred between two nodes, the circuit is brought down. Since SVC is established on-demand, therefore, their trade-off is the cost versus availability as compared to PVCs.
Acronym for Data Link Connection Identifier, a DLCI is used to identify virtual circuit on frame relay network. DLCI are usually locally significant means that DLCI is significant between the frame relay DCE and DTE interfaces. Figure-1 depicts the DLCI assignment.
FIGURE-1: Frame Relay DLCI Assignment
Local Management Interface (LMI)
LMI is signaling protocol developed by the Cisco and others. It is used to exchange messages and information on the state of connection. Using LMI, both end of the connection; determine the state of PVC and DLCI value defined, and whether the end node is active. The state information exchanged out-of-band using reserved DLCI value (0 and 1023). The supported LMI types on Cisco routers include: Cisco, ANIS Annex-D and Q933a Annex-A. LMI is not end-to-end, it used between the frame relay switch and routers.
ANSI and Q933A use DLCI 0 and Cisco uses 1023 for LMI messages.
LMI can be learned dynamically or statically configured. No configuration is required to dynamically detection LMI.
Step-1: configure terminal
Step-2: interface serial
Step-3: encapsulation frame-relay
Step-4: frame-relay lmi-type