642-642 – QoS Video Training – The Big Picture – QoS Design Strategies

642-642 – QoS Video Training – The Big Picture – QoS Design Strategies

Most focus is WAN Network Design not Campus LAN.

Campus Considerations


Campus link are usually less than 5% utilized from studies… (5Mbps in a 100Mbps FE LAN)

QoS can assist with DOS and Worm viruses. Policing can tag with a lower QoS label if traffic exceeds 5Mbps for example and then drop it.

QoS on router is done on software, switches do with hardware and software.

Ideally do all QoS tasks at the access layer! Let the ASICs do the work..

Goal: Trust Boundary marked as close to the source as possible.. retain marking across network all the way to router.. (ToS and CoS considerations)

Do not mark a the PC level however!! Only trusted devices such as IP Phones for example.


WAN Considerations

WAN uses software QoS

  • -Mark traffic early
  • -Older devices may be swamped!

“The Percentage Game”

  • – 25% of bandwidth left for best effort
  • -75% is normally the limit for all QoS apps… 25% must be left for router traffic (Routing Protocol, CDP etc..)
  • – LLQs 33% rule – Only reserve 33% for Priority Queuing
  • x Over-reservation affects DATA
  • x Over-reservations affects VOICE
  • Voice and Video should only consume 33% of the bandwidth. If you breach 33% then you need to review the size of you WAN link.
  • If you breach 33% then DATA applications maybe impacted.

Serialization Delay

  • How long from routers memory/buffer to the wire itself..
  • Goal is 10-15Ms delay per-hop
  • Required on links less than 768Kbps
  • LFI to combat this!


  • Most CPU intense QoS strategy
  • Use only on links less than 768Kbps



  • Router automatically tags all “mission critical” traffic with IPPres6/CS6. Routing updates/CDP etc….
  • PAK_Priority is a unchangeable, internal mechanism

TX-Ring Tuning

  • Hardware queue on the router…
  • Software Queue and Hardware Queue
  • Is tuned automatically in newer IOS versions
  • Typically  holds 32-64 packets by default
  • Reduce to 3 on slow (T1/E1) links.
  • FIFO queuing only..

show controllers Serial 0

tx_limited – TX ring size…

WAN Speeds – Best Practices

768Kbps or less

  • Video not recommended
  • LFI for VOIP
  • Carefully use cRTP
  • Reduce TX-RING to 3
  • 3 to 5 Traffic Classes Recommended

Links between 768Kbps – T1/E1 (2MBps)

  • VoIP and Video get LLQ priority
  • No LFI
  • Optional cRTP
  • Reduce TX-RING to 3
  • 3 to 5 Traffic Classes Recommended

High Speed Links (>T1/E1)

  • No LFI
  • No cRTP
  • 5 to 11 Traffic Classes Recommended