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




 show fram-relay pvc

 show frame-relay lmi


Local Access Rate

Local Access Rate or Access Rate defines the maximum transfer rate of an interface. The clock rate actually defines the access rate of an interface. It is configured on DCE end of the connection.



Configuration is straight forward.

Step-1: configure terminal

Step-2: interface serial

Step-3: clock rate ß in bps

Step-4: end



show interface serial


Committed Access Rate (CIR)

CIR defines the bandwidth for the virtual circuit guaranteed by the service provider under normal conditions. It is express in kilo-bits per second (kbps). Since CIR is usually lower than the actual access rate of the interface and routers transmit according to access rate, traffic shaping is required to hold the traffic in a (software) queue to accommodate the difference. Frame relay network may allow burst-able bandwidth also known as the Excessive Information Rate or Peak Information Rate. The provider may allow the traffic rate equal to CIR plus EIR if adequate bandwidth is available.


Acronym for Backward Explicit Congestion Notification and Forward Explicit Congestion Notification. Both are congestion avoidance techniques. BECN is used to notify the sender that congestion has occurred. FECN is used to inform the receiver of the traffic that congestion. Why used BECN and FECN? If user-A is send data to user-B across a frame relay network, and for some reason, congestion has occurred, the frame relay switch will send a BECN to user-A and FECN to user-B. User-A on receiving BECN, will apply flow control (by informing the upper layer protocols) and slow down its sending rate. User-B on receiving the FECN message, will inform the upper layer protocols to expect delay.

Discard Eligible (DE) Bit

The DE-bit is also a congestion management technique used by frame relay. Set by the frame relay switch when the current traffic rate exceeds the allowed CIR. In case congestion occurs, the packet marked with DE-bit can discarded at any point in the frame relay network. For example: the contracted traffic rate is 128kbps, but service provider is allowing its customers to send at PIR of 512kbps. In case congestion occurs, any packet above the CIR (that is 128kbps) can be marked with DE-bit and is now eligible to be dropped at any point in the frame relay network.