The DOCSIS WFQ Scheduler is an output packet scheduler that provides output scheduling services on both WAN uplink interfaces
and DOCSIS downstream interfaces.
Finding Feature Information
Your software release may not support all the features that are documented in this module. For the latest feature information
and caveats, see the release notes for your platform and software release. The Feature Information Table at the end of this
document provides information about the documented features and lists the releases in which each feature is supported.
Hardware Compatibility Matrix for the Cisco cBR Series Routers
The hardware components that are introduced in a given Cisco IOS-XE Release are supported in all subsequent releases unless
DOCSIS WFQ Scheduler
or softwareis needed to use the DOCSIS WFQ Scheduler feature.
Restrictions for DOCSIS WFQ Scheduler
The DBS feature is only
applicable to DOCSIS 3.0 downstream channel bonding.
DOCSIS WFQ Scheduler
The DOCSIS WFQ
scheduling engine is used to provide output packet scheduling services,
including absolute priority queueing, weighted fair queueing, minimum rate
guarantee, traffic shaping, and DOCSIS bonding group dynamic bandwidth sharing
on the Cisco cBR-8 converged broadband router.
The DOCSIS WFQ
Scheduler provides services on both WAN uplink interfaces and DOCSIS downstream
interfaces. The scheduling parameters on WAN uplink interfaces are configured
through the Modular QoS CLI (MQC). On cable downstream interfaces, queues are
created for DOCSIS service flows with parameters configured by DOCSIS
downstream QoS type, length, values (TLVs).
The default queue
size for the DOCSIS service flows (with bandwidth greater than 150 Mbps) is
based on the bandwidth on the cable downstream interfaces (see Table below).
Additionally, the queue limit for all service flows can also be adjusted using
cable queue-limit command, buffer size in service class or downstream buffer
queue size change, and the
cable queue-limit command do not affect the DOCSIS high
Table below is an
example of the queue size based on Annex B 256 QAM channels.
Table 1. Bandwidth,
Queue Sizes, and Queue Limits
The DOCSIS WFQ
Scheduler also allows significant enhancement to the queue scaling limits.
sections explain the DOCSIS WFQ Scheduler features:
The DOCSIS WFQ Scheduler feature supports the following types of queues:
Best Effort queues
Priority queues are
serviced with absolute priority over all the other queues. On DOCSIS downstream
interfaces, the priority queues are configured by DOCSIS applications that
request a priority service flow, for example, a packet cable voice service
flow. On WAN uplink interfaces, the priority queues are configured by the MQC
restrictions apply to priority queues:
Only one priority queue is
allowed per WAN uplink interface.
Only one priority queue is
allowed for low latency service flows created for each DOCSIS downstream
A CIR queue is
guaranteed to be serviced with at least the Committed Information Rate (CIR).
CIR queues are used to service DOCSIS service flows with non-zero minimum
reserved rates. If the offered load to a CIR queue exceeds its CIR value, the
excess traffic is serviced as best effort traffic.
conditions apply to BE queues:
On DOCSIS downstream interfaces, BE
queues are created by DOCSIS service flows that do not request a minimum
Each DOCSIS flow without a minimum
reserved rate uses its own BE queue.
DOCSIS defines a
set of quality of service (QoS) parameters, including traffic priority, maximum
sustained traffic rate, minimum reserved traffic rate, maximum traffic burst,
maximum downstream latency, and peak traffic rate.
service flows use the QoS parameters to specify the desired QoS. The downstream
policer and scheduler provides services such as traffic shaping, bandwidth
provisioning, traffic prioritization, and bandwidth guarantee.
The DOCSIS service
flow parameters are mapped to the packet queue parameters and provided with
appropriate QoS support for the packet queues to support the DOCSIS parameters
DOCSIS QoS parameters are supported:
The downstream channel bandwidth available to the best effort traffic,
namely the channel bandwidth minus the amount consumed by the priority traffic
and the CIR traffic, is allocated to the best effort service flows in
proportion to their DOCSIS traffic priorities. For example, if there are three
service flows sending packets at a particular moment over the same downstream
channel, and their DOCSIS traffic priorities are 0, 1 and 3, respectively,
their share of the channel bandwidth will be 1:2:4. To achieve this bandwidth
allocation, each service flow is assigned a value known as its excess ratio
which is derived from its DOCSIS priority. Table below shows the default
mappings of DOCSIS priority to excess ratio.
When traffic priority for a flow is not explicitly specified, a
default priority value of 0 is used as per the DOCSIS specification.
Table 2. DOCSIS Priority to Excess Ratio Mapping
DOCSIS Traffic Priority
Priority to Excess Ratio Mappings
This option is
introduced to configure custom priority to excess ratio mappings for downstream
service flows that override the default mappings listed in the above Table.
values are used only for new service flows that are created after the
configuration has been applied. All the existing service flows maintain their
previous excess ratio values.
The option to
configure priority to excess ratio mappings is available on a per downstream
forwarding interface basis and is applicable to legacy cable, wideband and
modular cable, and integrated cable interfaces.
downstream qos wfq weights command is used to configure the mappings.
Maximum Sustained Traffic Rate
The maximum sustained traffic rate (MSR) specifies the peak information rate of a service flow. The MSR of a service flow
is mapped to the shape rate of the packet queue. When the maximum sustained traffic rate is not specified or set to zero,
its traffic rate becomes limited only by the physical channel capacity set by DOCSIS specifications.
Minimum Reserved Traffic Rate
The minimum reserved traffic rate (MRR) specifies the minimum rate reserved for a service flow. The MRR of a service flow
is mapped to the CIR of the packet queue, which ensures the minimum amount of bandwidth a queue gets under congestion. When
the MRR is not specified, the CIR is set to zero as per DOCSIS specifications.
High Priority Traffic
High priority traffic flows are mapped to a Low Latency Queue (LLQ) on the data forwarding interface. The packets in LLQ are
serviced with absolute priority over other queues on the same interface.
The following service flows require high priority service:
Service flows with DOCSIS downstream latency TLV set to a value above zero. For example, PacketCable Multimedia Specification
(PCMM) voice calls.
PacketCable downstream service flows.
Service flows with Unsolicited Grant Service (UGS) type—non-PacketCable voice calls—upstream flows.
The DOCSIS WFQ
Scheduler supports the Enhanced Rate Bandwidth Allocation (ERBA) feature for
service flows. The ERBA feature allows cable modems (CMs) to burst their
temporary transmission rates up to the full line rate for short durations of
time. This capability provides higher bandwidth for instantaneous bandwidth
requests without having to make changes to existing service levels in the QoS
The DOCSIS WFQ
Scheduler allows each service flow to have one dedicated queue. When ERBA is
enabled for the service flow, the peak rate is implemented as the queue shape
rate within the scheduler, while the maximum sustained rate is set as the token
bucket refill rate. When ERBA is turned off, the burst size and the peak rate
value are not used.
The maximum traffic
burst parameter is used to control a service flow burst duration, to burst up
to the channel line rate or a configured peak rate, when it is within its
maximum burst size allowance. On the Cisco cBR-8 Converged Broadband Router,
cable ds-max-burst command is used to control this
The ERBA feature
is not applicable for high priority service flows and multicast service flows.
summarizes the ERBA support for the Cisco cBR-8 router.
Table 3. Enhanced Rate
Bandwidth Allocation Support for the Cisco cBR-8 Router
Token Bucket Size
Sustained Traffic Rate (unused)
computed internally by CMTS (unused)
Sustained Traffic Rate
ERBA-Enabled Service Flow
Sustained Traffic Rate
Traffic Burst TLV
about ERBA support on the Cisco CMTS routers, refer to Using Enhanced Bandwidth
Rate Allocation (ERBA) Support for DOCSIS 1.0 Cable Modems at the following
DOCSIS 1.1 for the Cisco CMTS
option of the
cable ds-max-burst command allows you to specify the
peak rate an ERBA-enabled service flow can use. The
value is a global value and is applied to all service flows created after the
configuration of the
cable ds-max-burst command. The default value of the
If the DOCSIS 3.0
TLV 25.27 is specified for a service flow, the
value is set as the TLV value. However, if ERBA is not turned on for a service
value is ignored.
The peak-rate value can also be configured through cable service class command which forms part of the service class template. During modem
registration or Dynamic Service Addition (DSA) operation, the service class name TLV 25.4 is sent to create the static or
dynamic downstream service flow that matches the service class template. These downstream service flows are created with a
specific peak-rate . If the peak-rate is not specified in he cable modem's configuration file, then the peak rate specified by the cable ds-max-burst burst-threshold thresholdpeak-rate peak rate command is used.
The option to specify peak rate in the cable ds-max-burst command is not available on the Cisco cBR Series Converged Broadband routers.
If a service flow
has both service class and TLV 25.27 defined
value specified in the TLV is used.
Some of the DOCSIS
1.x and DOCSIS 2.0 cable modems, which are not fully DOCSIS 1.x or DOCSIS 2.0
compliant, may fail to come online when they receive TLV 25.27 from the Cisco
CMTS during registration. In order to overcome this you can configure the
cable service attribute withhold-TLVs command with the peak-rate keyword to restrict sending of this TLV
to non-DOCSIS 3.0 cable modems.
Downstream Bonding Support with Bonding Group Dynamic Bandwidth Sharing
introduces the concept of downstream channel bonding. Each Bonding Group (BG)
is made up of a collection of downstream channels, which can be used by one or
more bonding groups. Each downstream channel can also serve as a primary
channel in a MAC domain and carry non-bonded traffic, while being part of a BG.
Prior to DOCSIS 3.0
standards, the downstream service flows were associated with a single
downstream interface, which in turn corresponded to a physical downstream on an
RF channel. In DOCSIS 3.0, the downstream service flows are associated with the
downstream bonding groups. These bonding groups can use multiple downstream RF
DBS is the dynamic
allocation of bandwidth for wideband (WB) and integrated cable (IC) interfaces
sharing the same downstream channel. Due to the channel sharing nature of the
bonding groups, the bandwidth available to bonding groups or non-bonded
channels is not fixed. The bandwidth depends on the configuration and the
traffic load on the WB or IC.
are implemented as WB interfaces and non-bonded channels as IC interfaces.
In the DBS mode,
the bandwidth of the shared RF channels is dynamically allocated among the WB
and IC interfaces. The DBS enables efficient use of the underlying RF channel
bandwidth even in the presence of high burst traffic. The DBS is configured at
the WB or IC interface level. By default, bandwidth for a WB or IC channel is
statically allocated (non-DBS).
the custom excess ratios for 8 priorities:
values are used only for new service flows and not existing ones.
interface configuration mode and returns to privileged EXEC mode.
Downstream Queues Information
To verify the
downstream queue information for a modem, use the
show cable modem
To check queue
stats of all queues on an Integrated-Cable or Wideband-Cable interface, use the
show cable dp queue interface command.
sections provide references related to the DOCSIS WFQ Scheduler feature.
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Use Cisco Feature Navigator to find information about the platform support and software image support. Cisco Feature Navigator
enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature
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The following table lists the software release in which a given feature is introduced. Unless noted otherwise, subsequent
releases of that software release train also support that feature.
Table 4. Feature Information for DOCSIS WFQ Scheduler
DOCSIS WFQ scheduler
Cisco IOS XE Everest 16.6.1
This feature was integrated into Cisco IOS XE Everest 16.6.1 on the Cisco cBR Series Converged Broadband Routers.