Frame Relay Virtual Circuit Parameters - Page 2

By Cisco Press | Posted Jan 22, 2002
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Figure 15-12 illustrates the process that each Frame Relay switch runs upon receipt of a frame for transmission.

Figure 15-12: Frame Relay Data Frame Transmission Flowchart
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(Click image for larger view in a new window)

DE requires that the Frame Relay network interpret, and in some cases act on, the DE bit. Some networks take no action when the DE bit is set, and other networks will use the DE bit to determine which frames to discard. Often the DE bit is used to determine which frames should be dropped first or which frames have lower time sensitivity.

DE lists can be created within the Cisco routing device to identify the characteristics of frames eligible for discarding. DE groups can be specified to identify the DLCI that is affected.

When a DE frame is discarded, it is up to the upper-layer protocols, such as TCP, to determine the loss and effect corrective actions as determined by the protocol's data correction and retransmission algorithms.


NOTE:   Within the Cisco IOS, the command used to define a DE list specifying the frames that can be dropped when the Frame Relay switch is congested is as follows (this command is entered in global configuration mode):
  frame-relay de-list list-number {protocol protocol | 
  interface type number} characteristic
For example, the following command specifies that IP packets larger than 512 bytes (including the 4 byte Frame Relay encapsulation) will have the DE bit set:
  frame-relay de-list 1 protocol ip gt 512
DE lists can be created based on the protocol or the interface, and on characteristics such as fragmentation of the frame, a specific TCP or User Datagram Protocol (UDP) port, an access list number, or a packet size (Layer 3 maximum transmission unit (MTU). To define a DE group that is specifying the DE list and is DLCI-affected, the following command is used in interface configuration mode:
  frame-relay de-group group-number dlci
For example, the following command specifies that group number 3 will be used for DLCI 170:
  frame-relay de-group 3 170

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