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Frame Relay Traffic Shaping 271
The information in Table 9-2 can be easily misunderstood. For instance, consider the details
around a 128-kbps Frame Relay link:
·
The CIR would have a value of 128,000 bps, but that's not necessarily the guaranteed
throughput value provided by the telco.
·
MinCIR is usually the guaranteed value and it defaults to one-half of CIR, so MinCIR
would be 64,000 bps.
·
The Bc is the CIR
ч 8, so it would be 16,000 bps.
·
The byte increment is Bc
ч 8, so it should be 2000 bps.
·
Finally, the Tc would be 125 ms, which is where it should normally sit.
All parameters might or might not be utilized in the course of traffic shaping. The parameters
are more related to Backward Explicit Congestion Notification (BECN) response network
congestion. BECN is explained more thoroughly in the next section of this chapter.
FECN and BECN
Frame Relay's traffic shaping capabilities rely largely on the capability to detect congestion. To
that end, Forward Explicit Congestion Notification (FECN) and BECN are implemented in
today's Frame Relay networks.
If a Frame Relay switch senses congestion in the network, it sets the FECN bit to 1 in the Frame
Relay header for traffic moving toward the destination device. This number indicates a
congestive situation. Once received, the destination device flips the BECN bit in return traffic
to the source. This informs the source device of the congestion in the network and that it should
reduce the transmission rate.
Once congestion has been experienced, any traffic moving through the network in violation
of the negotiated CIR that has been flagged as Discard Eligible (DE) can be dropped.
Retransmission of dropped traffic is left to the Layer 4 protocol of the end devices (for
example, to TCP).
If the router receives BECNs during the current time interval, it decreases the transmission rate
by 25 percent. The rate continues to drop with each BECN (limit one drop per time interval)
until the traffic rate gets to the minimum acceptable incoming or outgoing committed
information rate (MINCIR). At this point, the decline in throughput is halted.
Once the traffic rate has declined involuntarily, it takes 16 time intervals without the receipt of
BECNs to start to increase traffic flow again. Traffic increases by (Be + Bc)
ч 16, or more
accurately, the byte limit that shows up when the show frame-relay pvc command is divided
by 16.
It takes significantly longer to get back up to CIR than it did to drop. To shorten the length of
time, set Be to a value seven times the value of Bc. Continuing the example from Table 9-2,
note that the Bc is 16000 bps. If the Be is set to 7(16,000) = 112,000, the traffic increase is