IBM SC30-3681-08 manual Orphan Permanent Virtual Circuit CIR, Committed Burst Bc Size

Using Frame Relay

switched-virtual-circuitcon®guration command. You can also dynamically change

the CIR with the set circuit console command. You can also set the default CIR for

all Frame Relay circuits on this interface using the set CIR-defaultscommand.

Some Frame Relay switches allow a value of 0 to be con®gured for CIR. When CIR

is equal to 0, little or no bandwidth is reserved in the Frame Relay network

backbone for the VC, and the VC's traffic uses non-reserved bandwidth.

Orphan Permanent Virtual Circuit CIR

The router assigns a CIR to orphan circuits based on the CIR defaults con®gured at

the interface level. If you are relying on the orphan circuit to route important data

and the CIR, Bc, and Be values from the network provider are different from the

values con®gured at the interface level, it is recommended that you de®ne a PVC

instead of an orphan circuit. Doing this, you can assign a CIR that the network

commits to support.

Committed Burst (Bc) Size

The committed burst (Bc) size is the maximum amount of data (in bits) that the

network commits to deliver during a calculated time (Tc) interval. The Tc is equal to

the Bc divided by the CIR (Tc = Bc / CIR). If you con®gure 0 for CIR, Frame Relay

uses a value of 1 second for Tc..

For example, if you set a VC's CIR to 9600 bps and the committed burst size to

14 400 bits, the time period is 1.5 sec. (14 400 bits / 9600 bps = 1.5 sec). This

means that the VC is allowed to transmit a maximum of 14 400 bits in 1.5 seconds.

Note: The minimum Tc supported by FR is .1 second.

This parameter is important because of the relationship between the committed

burst size and the maximum frame size. If the maximum frame size in bits is

greater than the committed burst size, the network may discard frames whose size

exceeds the committed burst size. Therefore, the committed burst size should be

greater than or equal to the maximum frame size. It should also equal the burst size

set up with the network provider.

Use the add permanent-virtual-circuit,change permanent-virtual-circuit,add

switched-virtual-circuit or change switched-virtual-circuit con®guration

commands to set the committed burst size. The set circuit console command can

be used to dynamically change the committed burst size. You can also set the

default committed burst size for all Frame Relay circuits on this interface using the

set CIR-defaults command.

The device assigns orphan circuits a committed burst size based on the default you

set with the set CIR-defaultscommand. If you con®gure 0 for CIR, then the

committed burst (Bc) size also equals 0.

Excess Burst (Be) Size

The excess burst (Be) size is the maximum amount of uncommitted data the router

can transmit on a PVC in excess of the Bc during the Tc (Tc = Bc / CIR) when CIR

and Bc are nonzero. When CIR = 0, Frame Relay used a value of 1 second for Tc.

Chapter 30. Using Frame Relay Interfaces 397