Sunday, 5 February 2012

QoS - The Basic Steps

With any QoS configuration on a Cisco router there are only 5 steps involved. In any deployment a combination of these basic five steps can be used.

The most important thing to note is that QoS only comes into play in two situations:
  1. Slow speed links (ie less 2mbits) (like the edges of your network)
  2. Links that are reaching congestion
 In all other cases the configuration below does not really help.

Step 1 - Classification and Marking

Classification involves selecting some specific interesting traffic on which we can perform some actions.  The following methods can be used to classify traffic:
  • ACL's
  • NBAR
  • IP Addresses
  • MAC Addresses
  • Input interface
Marking means that you tag this traffic such that other downstream devices can also perform any actions they need to. Tagging can be done either at the layer three level (ie modifying the IP Packet) or at the layer two level (ie modifying the frame).
  • Layer 3 Tagging
    • IP Precedence
    • DSCP
  • Layer 2 Tagging
    • 802.1p Class of Service (COS) - Vlan Ethernet frames
    • ISL
    • ATM Cell Loss Priority (CLP)
    • Frame Relay Discard Eligible (DE) bit
    • MPLS Experimental bits
Step 2 - Congestion Management (ie Queuing or Scheduling)
In this step you basically stuff the classified and/or marked traffic into various queues so that you can select the traffic you want to send first while everything else waits. The follow types of queuing methods are available:
  •  Priority Queuing (PQ) - provides 4 queues
  • Custom Queuing (CQ) - provides 16 queues
  • Weighted Fair Queuing (WFQ) - provides 4096 queues (all managed automatically)
  • Class based WFQ (CBWFQ) - provides 64 queues
  • Low Latency Queuing (LLQ) - provides 64 queues same as CBWFQ
  • Modified Deficit Round Robin Queuing - provides 8 queues
Step 3 - Shaping and Policing
If you indeed have more traffic that the link can handled then you need to decide what to do with this.
  • Class Based Policing - At what threshold you need to start dropping traffic
  • Class Based Shaping - Buffer the traffic so that you can try move the peak traffic into gaps
Step 4 - Congestion Avoidance
These set of tools try to avoid congestion happening in the first place by automagically dropping traffic earlier so that congestion is never hit or sending messages back to the source to slow down
  • .Weighted Random Early Detect (WRED)
  • Explicit Congestion Notification
Step 5 - Link Efficiency
How you can squeeze more out of you the link you have.
  • Payload compression
  • RTP Header Compression (Class based)
  • TCP Header Compression
  • MLPPP Fragmentation and Interleaving (reducing transmit times for small packets)
  • Frame Relay and ATM Fragmentation and Interleaving

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