Command/Response (C/R)
This field’s use is not defined within the Frame-Relay standards and the field is
passed transparently across the network.
Extended Address
This version of FR does not support extended addressing.
Forward Explicit Congestion Notification (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.
Youcan configure the device to slow down data transmission in the direction from
which it receives a FECN using the enable throttle-transmit-on-fecn command.
Youcan also set the BECN bit in data frames sent to the originator of the FECN
using the enable notify-fecn-source command.
APPN High Performance Routing (HPR) uses detection of this bit set to allow Rapid
Transport Protocol’s adaptive rate-based flow 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 Notification (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-defined CIR when CIR or
congestion monitoring are enabled. The CIR for a PVC is supplied by the FR
service provider and is configured using the add permanent-virtual-circuit
command.
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. Toidentify traffic that is discard
eligible:
1. Configure BRS on the Frame Relay interface and any FR circuits that has traffic
that you are making discard eligible.
2. Assign a protocol or filter to a BRS traffic class using the assign command. You
specify whether the DE bit should be set on for this protocol or filter traffic.
User Data
This field contains the protocol packet being transmitted. This field 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 defined in RFC 1490.
Frame Check Sequence
This field is the standard 16-bit cyclic redundancy check (CRC) that HDLC and
LAPD frames use. This field detects bit errors occurring in the bits of the frame
between the opening flag and FCS.
Using Frame Relay
Chapter30. Using Frame Relay Interfaces 393