Having a Hard Time Getting QoS

Does anyone know of a great article on the web that explains QoS.
I've read some of the Cisco.com articles but either falling asleep half way through or smoke is coming from my ears.

I'm after something that starts from scratch.
I cant seem to find what the DSCP's mean either like AF31 or EF.

and if someone could explain the below snippet from my config that would be great also.
The policer line is where i really get stumped.

policy-map QOS-SCHEDULER-S
class PREMIUM
priority 512
class ENHANCED
police 1024000 conform-action set-prec-transmit 2 exceed-action set-prec-transmit 0 class ENHANCED-SIGNALING
bandwidth 32
class class-default
set ip precedence 0
fair-queue

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

dtlokee

mattsthe2 wrote:

Does anyone know of a great article on the web that explains QoS.
I've read some of the Cisco.com articles but either falling asleep half way through or smoke is coming from my ears.

I'm after something that starts from scratch.
I cant seem to find what the DSCP's mean either like AF31 or EF.

A DSCP value is simply a tag, it can mean whatever you want it to mean. The DSCP field has replaced the IP precedence in the Type of Service byte in IP header. IP precedence used the 3 most siginficant bits giving a total of 8 values, dscp uses the 6 most significant bits allowing for 64 different values. The value is broken into 2 parts, the calss selector and the drop probability, in this case AF31 would have a class selector of 3 and a drop probability of 1 (least) where as AF33 would be class 3 drop probability 3 (greatest chance to drop within the class).

EF is a special class that is designed not to be dropped and should be dequeued first (this needs to be configured with something like LLQ

mattsthe2 wrote:

and if someone could explain the below snippet from my config that would be great also.
The policer line is where i really get stumped.

policy-map QOS-SCHEDULER-S
class PREMIUM
priority 512
class ENHANCED
police 1024000 conform-action set-prec-transmit 2 exceed-action set-prec-transmit 0
class ENHANCED-SIGNALING
bandwidth 32
class class-default
set ip precedence 0
fair-queue

This particular police statement is known as a mark down policer, it dosen't drop the traffic that is in excess it remarks it with a lower IP precedence and transmits the traffic. The excess traffic is then treated differently by succesive hops because of the lower precedence. In this case the policer doesn't do much it wouul be the configuration on the next hop devices that would then place the lower priority traffic in different queues which could lead to the traffic being dropped.

mattsthe2

After every post you seem to repsond it in such a simple bitsize chuncks that everyone can understand - thanks.

So in that Police line why are we using IP Prec when you said DSCP seems to be the new way forward.

Because in our class maps we specifiy DSCP's (see below)

class-map match-all ENHANCED
match access-group name ENHANCED
class-map match-any BASIC
match any
class-map match-all PREMIUM
match ip dscp ef
class-map match-any ENHANCED-SIGNALING
match ip dscp cs3
match ip dscp af31

mwgood

This isn't exactly a great web article recommendation - I think it might be difficult to encapsulate QOS in an article.

Two sources I can recommend -

1) Jeremy Cioara QOS CBT Nugget
2) Wendell Odom's QOS Book

dtlokee

I would second the Odom QoS book.

They could be using IP precedence because of the next hop devices but that doesn't really matter in that DSCP values will be interpreted by IP precedence devices using only the top 3 bits. So in the case of AF31 it will convert to 011010 which is decimal 26 ((3 * 8 = 24) + (1 * 2 = 2) so the IP precedence device will see this as IP precedence "3" (the three most siginificant bits)

Ferret999

I would recommend reading the ONT cert guide and then the proper QoS guide. The ONT cert guide is a lot thinner and is easier to get through if you are learning from scratch. Also I would try the ONT CBT Nugget and the QoS CBT Nugget.

MACattack

Well at first it is weird knowing QOS like DSCP code, tag, TOS etc.. but as you read step by step knowing first the trunk fields how QOS is applied in the line then your next move is to know this codes like think of it what value should be in this fields.

Now, once you know this, undestand how Cisco wants you to apply this for what? what is your advantage if you use this likw you could use this in your IP phones, M-videos, important data need Priority.
Then learn the configuration, create first class map, in this think of match...like which traffic.
Then create Policy map think of this like a box and your class map will be something item.
Ofcourse you need a service think of this like DHL or and parcel and as you know there is differencet kine of services.

By the way better learn first this
Classification, Marking and Policy, Congestion Mgt and last but not the leat Congestion Avoidance.

Classification
Marking and Policy
Congestion Mgt --- FIFO, Priority Queing, custom Queing, WFQ, CB-WFQ and LLQ which is mix of Priority Que and CBWFQ.

If you want I can send you my notes about this, QOS is soooo COOL if you know how to configure and imagine you can now CONTROL your traffic not just like simple ACL,
Specially NBAR where can read traffic from layer 4 adn 7 of OSI layer.

Anyway, I just gave you a simple way to learn QOS... hope this helps..

mattsthe2

Ferret999 wrote:

I would recommend reading the ONT cert guide and then the proper QoS guide. The ONT cert guide is a lot thinner and is easier to get through if you are learning from scratch. Also I would try the ONT CBT Nugget and the QoS CBT Nugget.

Would this be the one??

ONT cert guide:

http://www.amazon.com/CCNP-Official-Exam-Certification-Guide/dp/1587201763

Even though the Odom book has had some very good review at least the above book goes toward my CCNP.

Thoughts?

Turgon

mattsthe2 wrote:

Does anyone know of a great article on the web that explains QoS.
I've read some of the Cisco.com articles but either falling asleep half way through or smoke is coming from my ears.

I'm after something that starts from scratch.
I cant seem to find what the DSCP's mean either like AF31 or EF.

and if someone could explain the below snippet from my config that would be great also.
The policer line is where i really get stumped.

policy-map QOS-SCHEDULER-S
class PREMIUM
priority 512
class ENHANCED
police 1024000 conform-action set-prec-transmit 2 exceed-action set-prec-transmit 0 class ENHANCED-SIGNALING
bandwidth 32
class class-default
set ip precedence 0
fair-queue

Try the Cisco QoS book by Wendall Odom for starters. QoS is a very involved subject with multiple mechanisms all with pros and cons. Learning QoS is a bit like becoming better at maths, you really have to work out solutions to a lot of problems. Before you can pretend to be proficient at QoS expect to spend at least a year covering the theory and comparing the results of configurations against particular requirements.