Information Technology Reference
In-Depth Information
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DE bit
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Frame Relay Error check
When a router sends a frame into the Frame Relay network, it can traverse many switches. If
the frame reaches a switch that is experiencing congestion, the switch sets the FECN bit in the
frame. The receiving router receives the frame with the FECN bit set. It's up to the upper-layer
protocols to determine what to do about the congestion. Sometimes, this bit is just ignored. The
BECN functions similar to FECN bits, but the BECN bit is set by the switch on frames heading
in the opposite direction of frames experiencing congestion. It is set on frames heading back to
the source of the traffic. Depending on the implementation, a flow-control scheme can be used
or the BECN can be ignored.
The DE bit marks frames that are eligible to be discarded if the Frame Relay switch becomes
congested. The Frame Relay network marks all frames in excess of the CIR as discard eligible.
The router can attempt to influence which packets are discarded by marking the frames as
discard eligible before they enter the Frame Relay network. The router can be configured to
mark frames based on traffic type or other parameters. In Example 5-1, any World Wide Web
traffic (on port 80) is marked as discard eligible on the egress of serial 0.1.
Configuration Defining Discard Eligible Traffic
Example 5-1
frame-relay de-list 1 protocol ip list 150
!
interface serial 0.1 point-to-point
ip address 192.168.1.5 255.255.255.252
frame-relay interface-dlci 100
frame-relay de-group 1 100
!
access-list 150 permit tcp any any eq www
Frame Relay also performs a cyclic redundancy check (CRC) to determine errored frames.
Frame Relay performs error checking and not error correction. Again, the upper-layer protocol
(TCP/IP) is responsible to correct errors or re-send packets.
Frame Relay Frame Format
Frame Relay uses Link Access Procedure for Frame Relay (LAPF) for frame format. LAPF is
yet another variation of the HDLC frame format; it supports Frame Relay. There is no control
frame, flow control, and error control, and no sequence numbers. Figure 5-7 shows the LAPF
frame format.
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