QoS Written 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


*** Theory ****

  • Best Effort – Is the default queuing method
  • IntServ – Integrated Services, reserves bandwidth for apps and is the 1st type of QoS. However inefficient as when the app is not using the bandwidth it is just wasted as other apps cannot use it. Not scalable
  • DiffServ – Divide traffic into classes and define how each class is treated.
  • IntServ vs DiffServ – Not all routers can support DiffServ and DiffServ does not provide the level of guarantee that IntServ does as it uses RSVP to carve out BW!.
  • QoS Methods
    • CLI – Legacy based and needs to be done interface by interface – not scalable!
    • MQC – Modular QoS CLI
      • Class maps – to classify
        • Config# class-map match-any JUNK_TRAFFIC
        • Config-cmap# match protocol // uses NBAR!
    • Policy maps – to mark
      • Config# policy-map MARKING
      • Config-pmap# class JUNK_TRAFFIC
      • Config-pmap-c# set precedence 0
  • AutoQoS – one command under the interface and it will automatically deploy QoS rules. It’s really good. Template on the network and same for all equipment!
  • QPM – QoS Policy Manager – GUI for cisco works
  • QoS Mechanisms
    • Classifications – Divide the traffic into classes
    • Marking – Mark as close to the source as possible
      • CoS – Marking at layer 2 e.g. switches
      • ToS – Marking at layer 3 e.g. router
        • IPPREC (0-7) – Legacy, maps CoS to ToS
        • DSCP (0-63) – Best way!
        • Queuing method Legacy
          • Queuing methods only exist if there is congestion
          • FIFO – Default
          • Priority Queuing – Suffers from queue starvation as it only service high priority queues first
          • Custom Queuing – Better than priority queuing,  can define how many packets / bytes to take from each queue via roud robin. Lessens starvation issue
          • Weighted Fair Queuing – prioritises the low volume senders over the high volume senders
          • Queuing methods – modern
            • Class-Based Weighted Fair Queuing
              • 16 queues  which has weighted fair queuing  of how many packets / bytes etc
  • LLQ
    • As soon as there is voice packets, that will go first with policing so no queue starvation and then everything else is CB-WFQ
    • Bandwidth
      • Bandwidth (CB-WFQ) statements should not be over 75% of the interface. Guarantees a minimum
      • Priority (LLQ) max bandwidth etc, polices it.
      • Remain – not often used, but uses 20% etc of the remaining percent e.g. 20% of 25%
      • WRED – combats taildrop which causes TCP synchronisation . It drops packets between the min value and max value, past the max it drops all of it.
      • Traffic Shaping – Outgoing only. Excess Traffic is queued. Typically used on FR and ATM networks
      • Traffic Policing – Incoming or Outgoing. Excess traffic is dropped or re-marked. Policing uses less memory buffers
        • Conforming traffic level  (CIR)-  e.g. transmits
        • Exceeding traffic level – e.g. remark the ip prec to say 0 and still transmit for WRED etc
        • Violating traffic level – e.g. drop the traffic!
        • Config-pmap-c# -Conform action transmit exceed-action drop
        • Link fragmenting and interleaving – operates at L2. Sometimes we may have large data packets in the hardware queue (not software queue) and until that is sent, voice packets will have to wait. What we can do is fragment the large data packet into smaller packets and mix them with voice packets so the voice packets do not have to wait so long. Useful for VoIP packets. Can do this on
          • Multilink PPP
          • FRF 12 (VoIP over Frae Relay)
          • L2 Compression / L2 Payload Compression – uses stacker, predictor and msoft.
            • Header Compression (Done at the interface and one side must be active)
              • TCP – IP and TCP header is compressed. Use with CBWFQ for good data transmissions.
              • RTP – IP (20 bytes), RTP(12 bytes) and UDP(8 bytes) headers are compressed to around 2 to 4 bytes. Use RTP HC and LLQ for voice, which is good



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