The bandwidth command is used to specify the minimum guaranteed bandwidth allocated for traffic matching
a particular class. Bandwidth may be defined as a specific value or may be set as a percentage of the interface
bandwidth.
By default, and without any QoS configuration, the default group of an interface is allowed a minimum
bandwidth of either 1 percent of the interface rate or 10 Mbps. This is applicable only on the egress.
Note
The actual bandwidth used is calculated in multiples of 256 kbps, the minimum bandwidth rate. The configured
value is rounded to the nearest multiple of 256 kbps. The value shown in the output of the running-configuration
shows the configured value as entered by the user.
If a percentage value is set, the accuracy that can be expected is 1 percent.
The bandwidth command can be used only within egress service policies. Use within an ingress policy is
not supported.
The bandwidth value takes into account the Layer 2 encapsulation that is applied to traffic leaving the
interface. For POS/SDH transmission, the encapsulation is considered to be 4 bytes. For Ethernet, the
encapsulation is 14 bytes; whereas for IEEE 802.1Q, the encapsulation is 18 bytes. The actual bandwidth
assigned to a class can be seen in the output of the show qos interface command.
Note
Be careful when specifying bandwidth guarantees close to 100 percent, because the Layer 2 encapsulation
considered does not include the entire Layer 2 header. This can lead to oversubscription, particularly in the
case of small packet sizes.
For bundled interfaces, bandwidth can be configured only as a percentage.
A policy map can have a single bandwidth statement per class. Both percentage and actual value bandwidth
configurations can be used within a policy map.
The bandwidth command does not specify how the bandwidth is to be shared. Instead it specifies how much
bandwidth is guaranteed per class, by setting the number of tokens that are assigned to the token bucket of a
particular class. For configured behavior to work correctly, you must ensure that the sum of the bandwidths
plus any priority traffic is not greater than the bandwidth of the interface itself. If the interface is oversubscribed,
unpredictable behavior results.
The bandwidth of the interface is set to be that of the physical interface, unless a hierarchical policy is defined
that reduces the bandwidth available to the traffic. The following example shows a hierarchical policy being
used to shape traffic to the specified value. The child policy then determines how the shaped bandwidth should
be apportioned between the specified classes:
policy-map parent
class match_all
shape average 1000000
bandwidth 1000000
service-policy child
policy-map child
class gold
bandwidth percent 20
class silver
bandwidth percent 40
class default
bandwidth percent 40
Cisco IOS XR Modular Quality of Service Command Reference for the Cisco CRS Router, Release 4.0
OL-23235-03 5
Quality of Service Commands on the Cisco IOS XR Software
bandwidth (QoS)
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