Figure
illustrates a Frame Relay environment in which different sites might
have different speeds for local-loop connections to the Frame Relay
cloud. Example
shows traffic shaping through a BECN example.
In this environment, the following
process can occur:
- Central site sends data to
router branch site.
- One of the switches within the
cloud determines that it is getting congested with traffic. In
this case, the congested switch sends a BECN packet to the central
router.
Note: Prior to the traffic shaping-over-Frame Relay
feature, the central router would note that the BECN was received,
but would not slow its transmission rate.
- At this point, packets begin
dropping within the switch that is encountering the congestion. As
previously discussed, this environment can cause problems,
particularly for latency-sensitive protocols such as SNA.
With the problem in this given process, the
solution is to enable the central router to dynamically fluctuate
the rate at which it sends packets, depending on the BECNs that it
receives. For example, if the central router begins receiving many
BECNs, it will reduce the packet transmit rate. As the BECNs become
intermittent, the router will increase the packet transmit rate.
Perform the following steps to configure
traffic shaping-over-Frame Relay BECN support:
- Define a map class and enter map
class configuration mode, as previously discussed.
- Make sure that BECN support is
enabled:
Router(config-map-class)#frame-relay
adaptive-shaping becn
BECN support is enabled by default.
When enabled, BECNs received from the network on this VC are used to
further regulate the output rate on the VC. As the frequency of
BECNs increases, the output rate is steadily reduced from peak to
average (equivalent of CIR). As congestion eases in the network
and the frequency of BECNs decreases, the output rate is allowed to
increase gradually to its configured peak.
- Enable both traffic shaping and
per-VC queuing for all VCs (PVCs and SVCs) on a Frame Relay
interface:
Router(config-if)#frame-relay
traffic-shaping
For VCs, where no specific traffic
shaping or queuing parameters are specified, a set of default values
is used.
- Associate a map class with an
interface or subinterface, as previously discussed:
Router(config-if)#frame-relay
class name
- (Optional) Apply the map class
to a specific DLCI for which a frame-relay map
statement
exists, as previously discussed:
Router(config-map-class)#frame-relay
interface-dlci
dlci [broadcast]
[ietf | cisco]