Cisco CCNA mock exam questions sample test – Question 165

What is the significance of the following BECN packet statistics?
A. The router is experiencing congestion in sending frames.
B. The router is experiencing congestion in receiving frames.
C. The Frame Relay mapping table is missing an entry.
D. The Frame Relay mapping table is corrupt.

Correct Answer: A

When frames arrived at a router with the Backwards Explicit Congestion Notification (BECN) bit set, congestion was encountered in the opposite direction from which the frame was
traveling. This bit is set by the Frame Relay switch. If an incoming packet has the BECN bit set, then this indicates congestion in outgoing packets, so the router will experience
congestion in sending frames.
When a Frame Relay switch encounters congestion, it will mark packets being sent in both directions on a PVC with either the Forward Explicit Congestion Notification (FECN) or the
BECN bit set. It will set the BECN bit on packets headed in the opposite direction of the congestion and FECN in the same direction as the congestion. When a packet with the FECN
bit is received by a router, it means there will be congestion when the receiving router receives packets.
A third type of marking is the Discard Eligibility (DE) bit. When this bit is set on a packet, it ensures that if congestion occurs and packets need to be discarded, the packet with the DE
bit set should be discarded first. ALL packets in excess of the committed information rate (CIR) are marked with the DE bit.
Frame Relay mapping tables have nothing to do with congestion in the Frame Relay network.
WAN Technologies
Describe basic QoS concepts
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