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Mechanisms Providing QoS
Class-based traffic shaping can be configured on any class and applied to any data path (interface
or DLCI) with the
that policy apply traffic shaping to a class. In the following example, class ring is shaped to 38.4
kbps, with a normal burst size of 15440 bytes.
XSR(config)#policy-map cbts
XSR(config-pmap<cbts>)#class ring
XSR(config-pmap-c<ring>)#shape 38400 15440
Traffic Shaping per Policy-Map
Traffic shaping can be applied per class or per policy map (multiple classes). The per policy-map
shaper effects all traffic transiting the policy. The aggregate bandwidth of all classes (one
configured with bandwidth and one with the priority command) within the policy is smaller or
equal to the shaper's average committed rate. Individual traffic flows share the bandwidth and are
prioritized within the shaper rate.
The per policy shaper may be useful in the following situations:
•
When a high- speed interface (Fast/GigabitEthernet) interfaces with a low-speed device such as a
modem where the downstream bandwidth is much smaller than the native Ethernet speed. The
QoS policy map is applied to the output Ethernet interface and traffic must be prioritized but
in accordance with the modem not Ethernet rate. To communicate this, the global shaper
should be applied to the policy map with the average rate equal to or smaller than the modem
speed.
•
On a virtual interface such as VPN which has no bandwidth associated with it. Applying the per
policy shaper to a virtual interface communicates the expected output bandwidth for that
interface to QoS and enforces bandwidth sharing and prioritization.
•
Over ATM VC interfaces configured with UBR on the same ATM port which may influence each
other's traffic. Applying the policy-map with shaper restricts the VC to a portion of the
available port bandwidth and enforces bandwidth sharing and prioritization.
The normal burst determines the biggest burst on the output interface and, as such, normal burst
should always be bigger than the biggest PDU size otherwise it may block traffic. The XSR will
produce a message concerning this situation but will not automatically increase the global shaper
burst.
The per policy shaper coexists with other QoS features such as shaping per traffic class, RED,
queue-limits, and others and does not affect their functionality. It is applied to the output port only
and has no effect if the policy map is configured on the input port.
In the following example, class cbts is shaped to 128 kbps. The overall cbts bandwidth is 128 Kbps
and if cbts is applied to the Ethernet interface the maximum egress rate would be 128 Kbps. Since
traffic classes share bandwidth within 128 Kbps, the priority class should not be configured more
than 75 percent of the shaper rate because if it sends with its maximum configured rate it may stall
all other traffic (including internally prioritized traffic such as RIP, OSPF and others).
XSR(config)#policy-map cbts
XSR(config-pmap<cbts>)#shape 128000
XSR(config-pmap<cbts>)#class d32
XSR(config-pmap-c<d32>)#priority high 30
12-8 Configuring Quality of Service
shape
command. In order to do so, you must define a traffic policy and within
Caution: When applied on a per policy basis, the shaper restricts output line bandwidth to the
average shaper rate. If the actual output line bandwidth is bigger than the shaper rate, the shaper
will stop traffic from using the entire output bandwidth.
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Troubleshooting
