Congestion Management

• FIFO Queueing
  • Simplest and easiest to configure.
    • Only parameter is queue-depth
  • Configuration
    • Disable Previous queueing strategy
      • ‘no fair-queue’
    • Define queue depth
      • ‘hold-queue out‘
  • Typically used as part of other solutions.
    • CBWFQ/HQF
  • Similar to ‘best effort’ traffic.

• Fair Queueing
  • Known as max-min scheduling.
  • Services multiple requests for a shared resource.
    • 1. Share resource equally
    • 2. Take excessive amounts
    • 3. Share excess equally among unsatisfied requests.
  • ‘Try and treat all traffic the same’.

• Weighted Fair Queueing
  • Max-min scheduling but not equal.
    • Allocate bandwidth per flow proportional to weight metric.
  • Flow is defined dynamically.
    • Src/Dst IP + Src/Dst Port + ToS Byte
  • Weight is IP Precedence + 1.

Configuration:

• ‘fair queue <CDT> <Queues>’
• ‘hold-queue out <max buffers>’

• CDT
  • Congestive Discard Threshold
    • Individual queue size threshold.
  • If number of flows > number of queues…
    • Flow collision occurs and queues are shared.

CBWFQ/HQF

• Allows defining of custom flows
  • Class definition using MQC Syntax
  • ‘bandwidth‘ keyword defines class’ ‘weight’
• Bandwidth is shared proportionally to weight
  • Relative sharing, not absolute reservation.

NOTE – Bandwidth does not work on subinterfaces.

• Every queue in Hierarchical QoS Framework (HQF) is FIFO
  • Includes ‘class-default’
  • Buffer-limit with ‘queue-limit’ command.
    • Global buffer limit with hold-queue out.
  • Can be turned into Fair-Queue
    • Command ‘fair-queue <FLOWS>‘
    • All flows are equal, no weighting.
    • Queue limit per flow is 1/4* queue-limit.
• Reservations
  • Absolute with ‘bandwidth [Kbps]’
  • Relative with ‘bandwidth percent [%]
    • Percent of interface ‘bandwidth’ setting
  • All bandwidths must sum to interface ‘bandwidth’
• Class-Default
  • Always guaranteed at least 1% of interface BW
    • max-reserved-bandwidth now deprecated.

Low Latency Queue (LLQ) in HQF:

• Priority Queue
  • Only one per Hierarchical QoS Framework (HQF) configuration.
    • Designated with ‘priority [x]’
    • Always emptied
  • Optionally policed to X Kbps only in times of congestion.
    • Congestion defined as having TX-Ring full.
  • Multiple classes can have priority
    • Share single queue but could be policed differently.
• Remaining Bandwidth
  • Commonly used with LLQ
  • Bandwidth remaining after LLQ allocations.
  • Command ‘bandwidth remaining x’
  • Calculated as Interface_BW – LLQ_BW