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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesClass-based queuing (CBQ) is a packet-scheduling and link-sharing method that groups network traffic into classes and allocates bandwidth through a hierarchy of queues. A classifier assigns packets to classes; a scheduler selects which class sends packets next. In hierarchical CBQ, a class may also use spare capacity elsewhere in the hierarchy.
How does class-based queuing work?
CBQ handles packets through distinct stages. Classification determines where a packet belongs; queuing holds it until service is available; scheduling chooses which queue to serve. Keeping these jobs separate helps explain what CBQ does—and what it does not do.
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- Classification: A classifier checks packet properties against configured rules and directs each matching packet to a traffic class. RFC 3290 describes a classifier as a functional element that selects matching and non-matching packets and forwards them along the appropriate router datapath. Read RFC 3290.
- Queuing: Packets wait in the queue associated with their class until the scheduler can serve them.
- Scheduling: The scheduler selects a queue according to its service discipline, such as its allocation or priority.
- Sharing capacity: In a hierarchical configuration, classes are nested under higher-level schedulers. When a class cannot meet its allocation from its own branch, a higher-level scheduler can check whether another branch has spare capacity.
What does the hierarchy mean for bandwidth?
A CBQ hierarchy represents how the link’s capacity is divided among traffic groups. RFC 3670 describes a top-level scheduler representing the full link capacity, with allocations divided among lower-level schedulers and queues. These allocations describe how capacity is shared; they should not automatically be read as hard caps.
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For example, imagine a link with an illustrative capacity of 100 units, divided between two branches with allocations of 60 and 40 units. If the second branch is using only 20 units, a class in the first branch that needs more than its 60-unit allocation may be able to use some of the otherwise unused capacity, subject to the configured hierarchy and available bandwidth. The numbers are only an example, not a measurement or a performance guarantee.
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This borrowing behavior is central to hierarchical CBQ: when a class lacks bandwidth for its allocation, schedulers higher in the tree may look for capacity elsewhere in the hierarchy. RFC 3670 describes this link-sharing model.
Does CBQ control delay or queue size?
Not by itself. CBQ determines how traffic classes receive service, but scheduling does not necessarily control how many packets build up in a queue. Large queues can still contribute to delay.
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Active queue management (AQM) addresses queue size and can help prevent unnecessarily high delay. RFC 7567 treats scheduling and queue management as complementary: a network can use a scheduler to prioritize, rate-limit, or isolate traffic while using queue management to control queues. A classifier, a packet-marking method, a scheduler such as CBQ, and AQM therefore have related but distinct roles. Read RFC 7567.
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What does CBQ mean in Linux?
In Linux, CBQ is available as the classful cbq queuing discipline (qdisc), configured through the tc traffic-control system. The Linux manual describes it as a hierarchical link-sharing model with shaping and prioritization capabilities. See the tc-cbq(8) manual.
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The documented Linux implementation estimates link idle time using dequeue timing and the configured underlying bandwidth. The manual notes two practical limitations: the actual bandwidth can be uncertain on paths such as PPPoE or PPTP, and coarse kernel timing can make shaping imprecise over short intervals. CBQ can tolerate substantial errors in bandwidth estimates, but the trade-off is coarser shaping. These are implementation caveats for Linux CBQ, not universal properties of every system that uses class-based queuing. Check the manual and behavior for the Linux version on your system before relying on particular configuration details.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How is CBQ different from other scheduling approaches?
CBQ is one option among per-flow and hierarchical scheduling approaches. RFC 7567 names WFQ, SFQ, DRR, CBQ, HTB, and HFSC as examples; their behavior depends on the implementation and configuration. A useful comparison focuses on whether an approach organizes allocations hierarchically, how it distributes excess capacity, whether it offers priorities or flow isolation, and what implementation constraints apply. Queue management remains a separate consideration unless the system explicitly combines it with scheduling.
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