Queuing Crib Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind.

Queueing                                                                                                                                                           

*** Theory ****

  • FIFO – not ideal for time sensitive traffic
  • WFQ – Weighted Fair Queuing – Allows flow/stream to go through. Runs default on serial connections with E1 or less.
  • CBWFQ – Class Based – Allows admin to decide what flows are transmitted first. Manual. Cant assign more than 75% of interface bandwidth as 25% is reserved for network control and routing
    • WFQ and CBWFQ can’t be running together
    • Tail drop – packet drop due to tail drop results in TCP senders reducing transmission rate., congestion is reduced, then transmission increases from all senders which means congestion again. This problem is known as  tcp global synchronisation
    • Weighted (WRED) / Random Early Detection (RED) – Helps combat TCP global synchronisation by using this instead of tail drop. RED uses IP Prec or DSCP to drop packets early before queue is full. WRED drops packet from other queues before priority queue. Ineffective against UDP!
    • Low Latency Queuing (LLQ) – Adds to CBWFQ. Allows to avoid Jitter. Used for VoIP
      • WRED and LLQ can’t work together
      • LLQ Policy = create extended access list > create a class-map and match access-list > create policy-map and assign the class-map to it > assign policy-map to interface
      • Priority Queuing – High, Med, Normal, Low.  

 

*** Commands ****

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