Frame Relay is a major topic on both your CCENT & CCNA 640-802 exams, & it is a major topic in today’s production networks as well!
Today, we will 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, & DE.
Frame Relay uses two different values to indicate that the WAN is becoming congested:
FECN - Forward Explicit Congestion
Notification
BECN - Backward Explicit Congestion Notification
As I am sure you can guess by the names, the main difference between the two is the
direction! But what direction? Glad you asked!
Let’s 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 is the direction in which the frames are traveling. Do you follow? 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 & FECN. Frames are occasionally dropped as a result of congestion, & frames with the DE bit set will be dropped before frames without that bit set.
Basically, setting the DE bit on a frame shows data that is considered less important than data without the DE bit set.
The FECN, BECN, & DE values
can be seen via show frame pvc.
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!
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, &
Ultimate CCNP Study Packages. You can also visit his blog, which is updated many times daily with new Cisco certification articles, free tutorials, & daily CCNA /
CCNP exam questions! For a FREE copy of his latest e-books, “How To Pass The CCNA” & “How To Pass The CCNP”, just visit the website! You can also get
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