Cisco CCNA And CCNP Certification Exam Training:

Frame Relay BECN And FECN

By Chris Bryant, CCIE #12933

BECNs and FECNs aren't just important to know for your Cisco CCNA and CCNP certification exams - they're an important part of detecting congestion on a Frame Relay network and allowing the network to dynamically adjust its transmission rate when congestion is encountered.

Now that you know what they are, you need to know what they do!   The Forward Explicit Congestion Notification (FECN, pronounced "feckon") bit is set to zero by default, and will be set to 1 if congestion was experienced by the frame in the direction in which the frame was traveling.  A DCE (frame relay switch) will set this bit, and a DTE (router) will receive it, and see that congestion was encountered along the frame's path.

If network congestion exists in the opposite direction in which the frame was traveling, the Backward Explicit Congestion Notification (BECN, pronounced "beckon") will be set to 1 by a DCE. 

In the following example, frames sent from Kansas City to Green Bay encounter congestion in the FR cloud.  A Frame Switch sets the FECN bit to 1.  In order to alert KC that it's sending data too fast, GB will send return frames with the BECN bit set.  When KC sees the BECN bit is set to 1, the KC router knows that the congestion occurred when frames were sent from KC to GB.

Frame Relay BECN Adaptive Shaping allows a router to dynamically throttle back on its transmission rate if it receives frames from the remote host with the BECN bit set.  In this case, KC sees that the traffic it's sending to GB is encountering congestion, because the traffic coming back from GB has the BECN bit set.  If BECN Adaptive Shaping is running on KC, that router will adjust to this congestion by slowing its transmission rate.  When the BECNs stop coming in from GB, KC will begin to send at a faster rate.

BECN Adaptive Shaping is configured as follows:

KC(config)#int s0
KC(config-if)#frame-relay adaptive-shaping becn

To view BECN and FECN statistics, run show frame pvc. This command also verifies BECN Adaptive Shaping.

R3#show frame pvc

< some output removed for clarity >

 input pkts 306           output pkts 609          in bytes 45566
 out bytes 79364          dropped pkts 0           in FECN pkts 0
 in BECN pkts 0           out FECN pkts 0          out BECN pkts 0
 in DE pkts 0             out DE pkts 0
 out bcast pkts 568        out bcast bytes 75128

shaping adapts to BECN
 pvc create time 01:26:27, last time pvc status changed 01:26:27

You need to know about FECN and BECN for your CCNA and CCNP certification exams, and for good reason - these are very important indicators of network congestion, indicators that allow you to do something about the congestion before it becomes a major issue.

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com