Frame Relay flow control

[filop]

Hello,

what does the Cisco router do, when it receives BECN or FECN bit from the Frame Relay network. Does it reduce the speed of outgoing data ? Please describe the situation, when the Cisco router has configured frame-relay traffic shaping and when it does not. Thank you.

Regards,

Peter

[thomas.chen]

FECN bits are used by the destination devices for flow control and BECN is used by the originating device for the flow control. They are functional whether you use traffic shaping or not.

More explanation below

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The FECN bit is part of the Address field in the Frame Relay frame header. The FECN mechanism is initiated when a DTE device sends Frame Relay frames into the network. If the network is congested, DCE devices (switches) set the value of the frames' FECN bit to 1. When the frames reach the destination DTE device, the Address field (with the FECN bit set) indicates that the frame experienced congestion in the path from source to destination. The DTE device can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow control may be initiated, or the indication may be ignored.

The BECN bit is part of the Address field in the Frame Relay frame header. DCE devices set the value of the BECN bit to 1 in frames traveling in the opposite direction of frames with their FECN bit set. This informs the receiving DTE device that a particular path through the network is congested. The DTE device then can relay this information to a higher-layer protocol for processing. Depending on the implementation, flow-control may be initiated, or the indication may be ignored.

The following doc explains in detail where you would use Frame-relay traffic shaping

http://www.cisco.com/warp/public/125/traffic_shaping_6151.html

HTH

[ckossmey · Accepted solution]

Minor correction.

FECN and BECN may be transmitted in the frame relay frames, but a Cisco endpoint will ignore them unless 2 things are true:

1) traffic shaping is configured on that serial interface

AND

2) "frame-relay adaptive-shaping" is configured in the frame-relay map-class. It is the default, so it may not show up in the config.

When a Cisco endpoint sees either a BECN or FECN, it will adjust it's transmission rate so that it eventually transmits with a max speed equal to the mincir configured in the map-class.

see the section "frame relay adaptive-shaping becn" in the link provided in the above response.

Regards,

Clay