IBM SC30-3681-08 manual Command/Response C/R, Extended Address, Discard Eligibility DE, User Data

Using Frame Relay

Command/Response (C/R)

This ®eld's use is not de®ned within the Frame-Relay standards and the ®eld is passed transparently across the network.

Extended Address

This version of FR does not support extended addressing.

Forward Explicit Congestion Noti®cation (FECN)

The FR backbone network sets this bit to 1 to notify the user receiving the frame that congestion is occurring for the PVC in the direction the frame is being sent. You can con®gure the device to slow down data transmission in the direction from which it receives a FECN using the enable throttle-transmit-on-fecncommand. You can also set the BECN bit in data frames sent to the originator of the FECN using the enable notify-fecn-sourcecommand.

APPN High Performance Routing (HPR) uses detection of this bit set to allow Rapid Transport Protocol's adaptive rate-based ¯ow and congestion control algorithm to adjust the data send rate. This algorithm prevents traffic bursts and congestion, maintaining a high level of throughput.

Backward Explicit Congestion Noti®cation (BECN)

The FR backbone network sets this bit to 1 to notify the user that the frames sent by this router for this PVC have encountered congestion. The router then initiates a throttle down to a rate equal to or less than the user-de®ned CIR when CIR or congestion monitoring are enabled. The CIR for a PVC is supplied by the FR service provider and is con®gured using theadd permanent-virtual-circuitcommand.

Discard Eligibility (DE)

The Frame Relay network may discard transmitted data exceeding CIR on a PVC. The DE bit can be set by the router to indicate that some traffic should be considered discard eligible. If appropriate, the Frame Relay network will discard frames marked as discard eligible which may allow frames that are not marked discard eligible to make it through the network. To identify traffic that is discard eligible:

1.Con®gure BRS on the Frame Relay interface and any FR circuits that has traffic that you are making discard eligible.

2.Assign a protocol or ®lter to a BRS traffic class using theassign command. You specify whether the DE bit should be set on for this protocol or ®lter traffic.

User Data

This ®eld contains the protocol packet being transmitted. This ®eld can contain a maximum of 8188 octets; however, the frame check sequence (FCS) can effectively detect errors only on a maximum of 4096 octets of data. The protocol data is preceded by a Frame Relay encapsulation header as de®ned in RFC 1490.

Frame Check Sequence

This ®eld is the standard 16-bit cyclic redundancy check (CRC) that HDLC and LAPD frames use. This ®eld detects bit errors occurring in the bits of the frame between the opening ¯ag and FCS.

Chapter 30. Using Frame Relay Interfaces 393