QoS really bugging me

lildeezul

ok so i am reading my BCMSN study guide, and i am really stressed out over this qos stuff.

like i get some stuff, but then other stuff is really vigorous. I really got to hit this stuff hard, becuase the CCIP course has a QOS exam....

so i just spent 150 bucks buying the CBT nuggets QOS vidoes series.. I think it will help me understand this QOS stuff, and help me with my ONT and QOS exam.

well off to church, and then more studying..

lildeezul

Ok these are some of the concepts i grabbed by re reading chapter 10 of the BCMSN self study guide.

QOS can use layer 2 and layer 3 classification methods via CoS (class of service) and DSCP,or IP precedence (old) respectively which belong to the ToS byte in a packet. CoS has 3 bits to use for classificatio in the 802.1q frame type. Which gives of a values 0-7.. value 6 and 7 are used for networking protocols such as EIGRP and other features that must have priority , becuase if not there will be no connectivity.value 5 is used for Voice, 4 for Video, 3 -2-1 for data, and 0 for FIFO ( first in first out/ Best effort)

IP-prec uses the 3 most significant bits in the TOS field which gives a value from 0-7.
DSCP ( diff service code points) use the 6 bits which gives 64 values from 0-63, and the last two bits are for ECN..

Classification is the method used to classify packets as a certain type of data, and arrange them into specific groups.. There are four ways to classify data.

1- trust modes via the MLS QOS TRUST DEVICE CISCO-PHONE (cdp used to find phone and trust DSCP values)

2- Manual classification based on DSCP values on incomming traffic ( MLS QOS TRUST DSCP/COS

3- Classification based on class maps, policy maps, and ACL

4- Classification using NBAR ( network based application recognition) THE COOLEST>

Marking is a method to "color" packets/frames so that the next hop doesnt have to deeply inspect the packet to classify the data. Marking makes classification easier becuase all the next hop has to do is look at the tag and say " aww this is voice data, or this is video data"
marking is done via ( MLS QOS DSCP/COS xxxx)

this is what i understand so far.

i am now reading about traffic shaping and policing...

tell me if you think im am getting this ok. ??? there is a lot to learn !

lildeezul

more knowlege to me


class maps are used for classification
and policy maps are used to determine what to do with the packet based on the class map
and you can apply policy maps via the SERVICE-POLICY interface config command.


i think im getting it..........but i am getting confused here....

i THOUGHT that classification is based on ingress traffic, but jeremy mentioned that you can use marking on ingress/egress frames..... i though marking would only be on egress frames/packets ( based on Cos or IPpred/dscp) to ease the process for other routers so they do not have to deeply inspect packets.

this above confuses me.. if marking is done on ingress frames then isnt that the same as classifying data ???? that confuses me.

oh wait.. i think i see where we would mark ingress...

lets say for examply off the router port their is a ip phone... if the IPphone is capable of marking thier own data on egress data, via cos 5 ( maps to dscp 40) but the router treats dscp 40 as low traffic priorty, then we would need to mark packets ingress that have a value of CoS 5 (DSCP 40) to a different value so it can use it as a internal DSCP.

would that be an example of this situation of marking ingress ??

lildeezul

ttt

APA

lildeezul wrote:

more knowlege to me


class maps are used for classification
and policy maps are used to determine what to do with the packet based on the class map
and you can apply policy maps via the SERVICE-POLICY interface config command.


i think im getting it..........but i am getting confused here....

i THOUGHT that classification is based on ingress traffic, but jeremy mentioned that you can use marking on ingress/egress frames..... i though marking would only be on egress frames/packets ( based on Cos or IPpred/dscp) to ease the process for other routers so they do not have to deeply inspect packets.

this above confuses me.. if marking is done on ingress frames then isnt that the same as classifying data ???? that confuses me.

oh wait.. i think i see where we would mark ingress...

lets say for examply off the router port their is a ip phone... if the IPphone is capable of marking thier own data on egress data, via cos 5 ( maps to dscp 40) but the router treats dscp 40 as low traffic priorty, then we would need to mark packets ingress that have a value of CoS 5 (DSCP 40) to a different value so it can use it as a internal DSCP.

would that be an example of this situation of marking ingress ??

Think of marking on ingress from a customer \ service provider point of view...

Say for example the customer is an enterprise and it is making use of the SP MPLS network - On egress the enterprise marks all it's traffic as high priority..... (DSCP 46, IPP 5 etc... What ever you want to use)

Now based on the trust boundary the SP should honour this value.... but what if the customer is not paying for high priority service from the SP... Why should the SP honor this???

So the SP on ingress can mark those packets as lower priority through it's network..... via the EXP field of the MPLS header... While leaving intact the IP header with the customers IPP or DSCP settings so upon exiting the MPLS network the customer still has their priority value in place as the SP only manipulated the MPLS header through it's network (I can explain in detail why the IPP or DSCP value isn't used here but at the moment it's out of the scope of this question... ....)

I don't think you should stress so much about QoS for the BCMSN..... I'm currently finishing up ONT and will get stuck straight into QoS for CCIP\CCVP upon finishing it.... The books for these two exams go into great detail about Cisco and QoS and you will grasp it no problems at all!!!!

Hope this helps

lildeezul

ahah haa thanks. I understand it now. Thanks for that enlightment.. Yeah i really want to learn this QoS stuff, its very interesting, and hard at the same time. I am also going to do the QoS exam for the CCIP at the same time as the ONT exam.

but based all the above information, does it seem like im getting these concepts down.

APA

ahah haa thanks. I understand it now. Thanks for that enlightment.. Yeah i really want to learn this QoS stuff, its very interesting, and hard at the same time. I am also going to do the QoS exam for the CCIP at the same time as the ONT exam.

but based all the above information, does it seem like im getting these concepts down.

Nothing worthwhile ever comes easily my friend.... otherwise everyone would be doing it

You definately seem like your grapsing the topics so far don't worry it will come to you if you keep at it...

I'm just wondering how my brain is maintaining all this info..... ;p Anywhose back to it! (and that's for both of us!!!!)