Cisco CCNA 640-802 CCENT Exam Training: Frame Relay BECN, FECN, and DE
Posted: Tuesday, November 20, 2007
by Chris Bryant CCIE 12933
The Bryant Advantage
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:
BECN - Backward Explicit Congestion Notification
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!
Lets assume Router A is sending frames to Router B. 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.
Well take a look at more important Frame Relay details in the next installment of my exclusive Cisco CCENT / CCNA certification exam tutorial series!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNA 640-802 exam, CCENT certification tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.
You can also visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions! For a FREE copy of his latest e-books, How To Pass The CCNA and How To Pass The CCNP, just visit the website! You can also get FREE CCNA and CCNP exam questions every day!
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