Internetworking Terms: Class-Based Weighted Fair Queuing (CBWFQ)

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A CB queuing mechanism (as opposed to flow-based WFQ) that reserves configurable bandwidth to each queue. Since a queue is pretty much Class-Based classified (packets are identified and queued according to CB possibilities), the term class is often a synonym of queue.

Queuing features:

  • Classification based on CB Markings, Access Control Lists (ACLs), ingress interface, RTP port ranges, Network-Based Application Recognition (NBAR), source/destination IP addresses, TCP/UDP ports, Experimental MPLSCOS, DSCP values and more.
  • Drop decision might be Tail Drop or Weighted Random Early Detection (WRED).
  • Up to 64 queues.
  • Each queue has a maximum number of 64 packets.
  • The scheduling logic within a single queue can be First-In First-Out (FIFO) or WFQ (Only within the queue called class-default, but the 7500 series routers support WFQ in all queues).
  • The scheduler takes the configured bandwidth values for each class, and makes sure each queue drains according to those values. Inactive queues' bandwidth is shared among active queues.

Configuration

The following commands are related to CBWFQ:

  match dscp (all other DSCP commands as well)
  class-map
  policy-map
  bandwidth (kbps/percent/remaining percent). Once the CB classification is done, this is the command that ignites CBWFQ strategy. Derives from the quantity set in the bandwidth interface command.
  queue-limit: sets max. number of packets for a class 
  fair-queue: enables WFQ in class-default
  random-detect dscp: enables WRED drop policy instead of tail drop

Verification:

  show policy-map: configuration details. If used with an interface provides statistics 
  show running-config

--Jose Mendez (CR-TAC) 07:10, 9 June 2010 (UTC)

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