Cisco CCENT / CCNA 640-802 Certification Exam Training :

Frame Relay FECNs, BECNs, and the DE Bit

By Chris Bryant, CCIE #12933

Frame Relay is a major topic on both your CCENT and CCNA 640-802 exams, and it's a major topic in today's production networks as well! Today, we'll take a look at three different Frame Relay values that help to indicate network congestion as well as decide what frames to drop due to that congestion - FECN, BECN, and DE.

Frame Relay uses two different values to indicate that the WAN is becoming congested:

As I'm sure you can guess by the names, the main difference between the two is the direction!  But what direction?  Glad you asked!

The frame relay cloud shown consists of multiple Frame Switches, but for clarity's sake, I'll only illustrate one.  If that switch encounters transmission delays due to network congestion, the switch will set the FECN bit on the frames heading for Router B, since that's the direction in which the frames are traveling.  The BECN bit will be set on frames being sent back to Router A.

When a frame arrives at a router with the FECN bit set, that means congestion was encountered in the direction in which the frame was traveling.

When a frame arrives at a router with the BECN bit set, congestion was encountered in the opposite direction in which the frame was traveling.

The Discard Eligible bit is considered a Frame Relay congestion notification bit, but the purpose is a bit different from the BECN and FECN.   Frames are sometimes dropped as a result of congestion, and frames with the DE bit set will be dropped before frames without that bit set. Basically, setting the DE bit on a frame indicates data that's considered less important than data without the DE bit set.

The FECN, BECN, and DE values can be seen via show frame pvc.

R1#show frame pvc

PVC Statistics for interface Serial0 (Frame Relay DTE)

              Active     Inactive      Deleted       Static
  Local          2            0            0            0
  Switched       0            0            0            0
  Unused         0            0            0            0

DLCI = 122, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0

  input pkts 30            output pkts 0            in bytes 2280
  out bytes 0              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 0          out bcast bytes 0
  pvc create time 00:07:45, last time pvc status changed 00:06:55

We’ll take a look at more important Frame Relay details in the next installment of my exclusive Cisco CCENT / CCNA certification exam tutorial series!

Five Minutes From Now, You Can Be Studying For CCNA Exam Success Just As Thousands Of Other CCNA Candidates Around The World Have - With Chris Bryant, CCIE #12933 With You Every Step Of The Way - With The Ultimate CCNA Exam Study Package!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com