CCIE - Internetwork Expert's Online Community

Hello,

a simple question....if we do nothing but to set precendence or DSCP of the transit traffic through node in CBWFQ(no bandwidth reservation), does it impact the overall traffic pattern in the topology? question arose because i couldnt fully understand the Qos PHB behaviour and it definately relates with..

regards,

Khurram 

If you use the Modular Quality of Service (MQC) to set precedence or DSCP and nothing more, technically you are doing Class Based Marking, and you are not performing Class Based Weighted Fair Queuing. 

Marking traffic is not enough to influence Quality of Service in your network. You must combine marking with Congestion Management tools like CBWFQ or LLQ and/or Congestion Avoidance tools like WRED in order to impact traffic patterns. 

The amazing Auto QoS feature actually manages to do all configurations automatically, but you have to make sure that AutoQoS is supported on all of your Cisco equipment in order for it to be effective. 

Thanks Anthnoy for clear understanding about C & M and its relation with Qos tools,

but i still have confusion about PHB , because Docs narrates that marking is related with PHB, so does PHB relates to something like local significance or am i detracking myself?

Khurram

Per hop behavior (PHB) is a way to standardize the marks. For example, if the entire world agrees to mark voice traffic with the Expedited Forwarding (EF) PHB, then we will all be able to recognize that traffic as voice. 

But notice, even though we might all be marking our voice traffic the same way, we still need to program our routers, switches, firewalls, etc., to treat this traffic with special priority. 

So i am just eloborating your said scanario further more, if Router-A marks the voice traffic with EF, so is there any need to re-mark by Router-B(next to Rouer-A) or it just needs to priortize marked traffic in order to give low latency through the whole transit. 

Thanks

You are correct - each hop must be programmed to provide the appropriate treatment to the marked traffic. 

Marking is recommended as close to the source of traffic as possible to provide the prioritized treatment throughout as much of the data path as possible. 

The great news is with Cisco IP Phones and voice traffic - the IP phone makes the voice packets at Layer 3 as EF and it also marks them at Layer 2 as CoS 5. Now it is up to an engineer to ensure these appropriately marked packets/frames are treated with the correct behavior. 

i have grabbed it fully now, Thanks

It is my pleasure! Thank you so much for choosing Internetwork Expert. 

You have inspired me to do a blog post on the Qos life of a packet as it travels from one internetwork to the next. Watch for it soon on blog.internetworkexpert.com 

Waiting for some more good stuff on blog