When configuring class-based WRED on Cisco routers, which WRED parameter is not user
configurable on a Cisco IOS XR but is user configurable on a Cisco IOS and IOS XE?
A.
the ingress or egress direction where the class-based WRED policy will be applied
B.
the maximum threshold
C.
the minimum threshold
D.
the mark probability denominator
Explanation:
Comparison of Cisco IOS QoS and Cisco IOS-XR QoS
The Cisco IOS-XR software implementation of QoS is basically the same as the QoS
implementation on Cisco
IOS software, with the following exceptions:
•On Cisco IOS-XR software, the bandwidth command can be configured only in egress policies.
•The following changes have been made to the class-map command on Cisco IOS-XR software:
–Supports 4K per logical router.
–Maximum number of match criteria configurable in one class map is eight.
•When a class is marked as high priority using the priority command on Cisco IOS-XR software,
werecommend that you configure a policer to limit the priority traffic. Limiting the priority traffic will
ensure that thepriority traffic does not starve all of the other traffic on the line card. Use the police
command to explicitlyconfigure the policer.
•On Cisco IO-XR software, only one conform-action, exceed-action, or violate-action command
can beconfigured at a time. To configure traffic policing, use the police command.
•On Cisco IOS-XR software, policy modifications cannot be made on existing policies. Use the
policy-mapcommand to remove the policy from all attached interfaces, delete the policy map, and
redefine a new policy.
•When configuring a policy map on Cisco IOS-XR software, the maximum number of classes
configurable inone policy map is 16, which includes both Level 1 and Level 2 classes. To configure
a policy map, use thepolicy-map command.
•When WRED is configured on Cisco IOS-XR software, the mark probability in the random-detect
command isnot configurable—it is always set to 1.
•When the random-detect exp command is used on Cisco IOS-XR software, the exponential
weightingconstant is not configurable and will be programmed automatically by Cisco IOS-XR
software.
•When access control lists (ACLs) are used in QoS class maps, the underlying deny or permit
actionsassociated with access control entries (ACEs) are ignored. ACEs are used as a
classification mechanism inorder to provide appropriate QoS behavior as specified in class maps.
Use ACLs that include ACEs with permitactions only.
RED traffic Profiles/Modes are used to determine the packet-dropping strategy and is based on the average queue length. The rate of packet drop increases linearly as the average queue size increases, until the average queue size reaches the maximum threshold.
IOS/XE use mark probability denominator – the fraction of packets that are dropped when the average queue depth is at the maximum threshold. If the denominator is 20, then 1 of 20 packets is dropped when the average queue is at the maximum threshold. For IOS XR, the mark probability denominator is 1 and is not configurable.
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