Frame Relay Traffic Shaping - Page 3

By Cisco Press | Posted Feb 7, 2002
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Derived Rates
FECNs and BECNs indicate congestion in a Frame Relay WAN and are specified by bits within a Frame Relay frame. FECN and BECN operation is as follows:

  • FECNs -- These are generated when data is sent out of a congested interface. FECNs indicate to a Frame Relay device that congestion was encountered along the transmission path to the destination. Traffic is marked with BECN if the queue for the opposite direction is full enough to trigger FECNs at the current time.
  • BECNs -- These notify the sending Frame Relay device to decrease the transmission rate. If the traffic is one-way only (such as multicast traffic), there is no reverse traffic with BECNs to notify the sending device to slow down. When a Frame Relay device receives a FECN, it first determines whether it is sending data. If the Frame Relay device is sending data along the return path of the FECN, this data will be marked with a BECN on its way to the other Frame Relay device. If the Frame Relay device is not sending data, it can send a Q.922 "TEST RESPONSE" message with the BECN bit set.

When an interface that is configured with traffic shaping receives a BECN, it immediately decreases, or throttles down, its maximum rate by a significant amount. If, after several intervals, the [throttled] interface has not received another BECN and traffic is waiting in the queue, the maximum rate slightly increases. This dynamically adjusted maximum rate is called the derived rate, which will always be between the upper bound and the lower bound that is configured on the interface.

Traffic Shaping Restrictions
FRTS applies only to Frame Relay PVCs and SVCs.

Figure 15-28 represents the traffic shaping process flow upon receipt of a frame for transmission.

Figure 15-28: Traffic Shaping Flowchart
Click image for larger view in a new window
(Click image for larger view in a new window)

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Our next segment from Cisco Press' Network Consultants Handbook will deal with Traffic Policing and Shaping.

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