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Qos Implement ion in LAN Environment

qosHi All,

We are planning to implement QoS on our network to increase the bandwidth for Voice traffic. So we have Cisco 3560 switch and a router where the WAN link is terminated.

1 ) If i trust the access port / cisco phone connected port by using CoS and the default cos-dscp value is 40 means ..then the policy map based on dscp value 46 ( EF ) in the router will not be effective ?

2 )  Also if i have so many trunks between the edge host and the router and and trusting the edge host with DSCP and in all the trunks i use trust Cos command will that alter the value by any chance at the router ?

3) Is there any mandatory case where we to need use mls qos trust CoS ?

4) using DSCP trust in the all ports is advisable ?

Regards,

Gan

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VIP Mentor

Hi Gan

 ) If i trust the access port / cisco phone connected port by using CoS and the default cos-dscp value is 40 means ..then the policy map based on dscp value 46 ( EF ) in the router will not be effective ? yes its the wrong mapping

change the map to be cos 5 -- 46 if your using cos , but best off to just use dscp at access and trunk
mls qos map cos-dscp 0 8 16 24 32 46 48 56

2 )  Also if i have so many trunks between the edge host and the router and and trusting the edge host with DSCP and in all the trunks i use trust Cos command will that alter the value by any chance at the router ?
don't use cos in trunk use dscp , either dscp/dscp or cos access and dscp at trunk and make sure mappings are right if you use cos at access ports so it maps to dscp correctly

3) Is there any mandatory case where we to need use mls qos trust CoS ? only on old switches/devices that don't understand dscp , you wnat to use dscp as it matches 6 bits its better


4) using DSCP trust in the all ports is advisable ?

yes cos is layer 2 dscp is layer 3 header , if the phones mark in dscp trust dscp right from access through trunks up to router whwre MQC will be to continue to honor markings

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Impressed !!!!

Relieved as i understood some basics clearly...

Final question before i sign off ... As we are trusting the DSCP values at the edge ports .... and for for voice we are marking as EF ..so by default its have the priority queuing in the Egress interfaces.... my question is then y we do policing and shaping WAN links side with some bandwidth .... Y can we do the same kind on priority queuing so the voice will have the priority ....

Guess i am confusing ..but what i want to know understand is this necessary to do policing / shaping after marking the packets as EF...

Regards,

Gan

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Yes I would and I do and it works fine , the phones by default should be marking with at source with EF and then your trust the DSCP value at access port and trunk , you can check by using a wireshark and spanning the port make sure you see that EF in the packet that's collected , you can dig into the packet and see EF as screenshot

Image result for wireshark EF

Guess i am confusing ..but what i want to know understand is this necessary to do policing / shaping after marking the packets as EF...

Qos isn't a fix all it just helps with critical traffic if there is congestion on the network , rarely you see heavy constant congestion on the lan but you will see it at your wan as bandwidth is not infinite , you may have a gig or 10gb lan but unless your a huge tech company its unlikely you will have that wan throughput and even if you do it wont be on every connection , so policing and shaping are important but at the wan side using MQC more than the lan  side where using MLS or Auto qos , sometimes you may have to tweak MLS though and Auto qos too if your seeing drops or having issues

In MLS there is a priority voice queue no 1 and you can also set priority-queue out under each voice interface which will make sure voice is serviced first before all other queues in MLS , this is optional and be careful using it as it services voice queue first it may starve the other queues temporarily, but if its only voice your really concerned about then I would use it

So is policing and shaping important it can depend on the network itself and what you want to make sure gets through , af41 and EF are usually what standard setups are concerned about voice/video and another reason its very important Is this is WAN link you don't own it , if you go over what you have been given they will cut that off on their side and they wont care what traffic they will just drop it , so its best practice to control it from your side make sure ISP doesn't do it :)

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