QOS resources and observations


  1. Cisco QOS Certification Guide. Link
  2. IOS 12.2 – QOS Configuration Guide. Link
  3. DSCP, AF, EF . Link
  4. QOS FAQ. Link
  5. CB-WFQ and CB-WRED. Link
  6. Difference between policing and shaping. Link


  • RTP is used for transfer of multimedia data, and the RTCP is used to periodically send control information and QoS parameters. Works on UDP.
  • “match protocol rtp audio”
    • Above command is equivalent to “match ip rtp 16384 16383”. Both commands match only even numbered ports from 16384 to 32767.
  • RTP for voice uses the range 16384 to 32767 (only even numbers)
  • RTP for video uses 49152 to 65535 (only even numbers)
  • RTP control packets (RTCP) uses odd numbers between 16384 to 32767.
  • “Access-list 100 permit udp any any range 16384 32767” matches both RTP audio and control packets.
  • Apply policy-map command on an interface which is configured as L2/L3.
  • Match commands in policy-map;
    • match bgp-index : to match via BGP “traffic-index” value set in a route-map.
  • “For auto-qos, configure IP address, bandwidth on an interface and enable CEF and disable any service policy configured on an interface. Command: “auto qos voip”
  • Seems we cannot allocate full bandwidth in a policy-map. At least 1% bandwidth should be available for default-class.
  • We should use only bandwidth or bandwidth percent alone inside all the class of a policy-map. No restriction with priority and bandwidth commands.
  • CAR ( Committed Access Rate) is cisco legacy traffic policing feature. No longer supported. Use “rate-limit” command on the interface mode.
  • “shape average 8000 1000” shows;
    • shape (average) cir 8000, bc 1000, be 1000
    • target shape rate 8000
  • “shape peak 8000 1000” shows;
    • shape (peak) cir 8000, bc 1000, be 1000
    • target shape rate 16000
  • PIR = CIR (1+ Be/Bc)
  • ‘service-policy” command can be configured on an interface configured with FIFO. No other configurations like WFQ, Random-detect configs are allowed.
  • “mls qos trust extend cos x” instruct the IP phone to rewrite PC traffic to cos value “x”.
  • “wrr-queue cos-map <queue_id> <threshold> <cos_values>”
  • “we must modify the switch default CoS-to-DSCP mappings, to map cos value of 3 to DSCP value of 24 (voice control) and cos value of 5 to DSCP value of 46 (voice payload), using command “mls qos map cos-dscp 0 8 16 24 32 46 48 54”
  • IP Prec 5 (101) maps to IP DSCP 101 000 (CS5 in DSCP term)
  • The default exponential weight factor in calculating average queue-depth in WRED is 9.
  • If the value of n becomes too high, WRED does not react to congestion. Packets are transmitted or dropped as if WRED were not in effect.
  • If the value of n becomes too low, WRED overreacts to temporary traffic bursts and drops traffic unnecessarily.
  • QoS Policy Propagation through Border Gateway Protocol (QPPB) allows you to classify packets by IP precedence based on BGP community lists, BGP autonomous system paths, and ACL.
  • If the “traffic-shape fecn-adapt” command is configured at both ends of the link, the far end will reflect received FECNs as BECNs in Q.922 TEST RESPONSE messages.
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