QoS - Cos & DSCP Value mapping

[Ganesan Palaniappan]

Hi 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

[Mark Malone]

Hi

you can use dscp on both uplink and access mls qos trust dscp or use cos on access and dscp on uplinks but if your devices understand dscp use that 

whats your mapping show can you post it should be this below for cos5-dscp46

mls qos map cos-dscp 0 8 16 24 32 46 48 56

                                     0 1  2   3   4   5   6

yes use it on all uplinks where possible , span the access port and the egress trunk port as well as a test make sure you can see that the voice packets are being marked correctly with EF in wireshark like below to confirm its working

[Ganesan Palaniappan]

Hi,

thanks for the reply ..My mapping shows Cos 5 - DSCP 40... Do i need to do this mapping globally in all the switches ?

Regards,

Gan

[Mark Malone]

did that automatically appear after enabling mls globally or was it manually entered like that ?

if you use dscp the whole way through it shouldn't matter as it doesn't need to be mapped but if your marking cos at access for layer 2  0-7 markings then yes change it , dscp maps at layer 3 0 -63

[Ganesan Palaniappan]

After enabling mls qos -- below is the output ...

Cos-dscp map:
cos: 0 1 2 3 4 5 6 7
------------------------------------
dscp: 0 8 16 24 32 40 48 56

So i need to map manually as 46. Do i need to use this in all the switches globally ?

Also is that mandatory..that i need to use trust CoS on the trunk and phone connected phones ?

Marking EF is only do explicated forwarding right .. what if i do policing like allocating bandwidth percent or priority ( i dont whats the difference again with bandwidth percent and priority )..what will do more by adding this ?

Because in my setup ..we are doing marking and setting the voice traffic as EF and not doing anything ..like bandwidth....

Regards,

Gan

[Mark Malone]

if your going to trust cos then you need to change the map to match cos 5 to dscp 46

if your trusting dscp at source access ports and trunks then no you shouldn't need to do anything but confirm dscp is working

Marking EF is only do explicated forwarding right .. what if i do policing like allocating bandwidth percent or priority ( i dont whats the difference again with bandwidth percent and priority )..what will do more by adding this ?

Yes only EF , that's layer 3 qos you wont see that in MLS which your deploying  , Policing bandwidth and priority statements all go in class maps , mls is queueing buckets per interface

Because in my setup ..we are doing marking and setting the voice traffic as EF and not doing anything ..like bandwidth.... You don't have to qos can be enabled multiple ways  it is recommended though voice is set with priority in class maps but again that's layer 3 usually at wan break out

[Ganesan Palaniappan]

Thanks Mark...

I am trusting all the ports with the DSCP only and not with CoS ..So extra configuration.... Will implement and let you know ....

how do i know the dscp is working .. sh mls qos interface command shows that ?

Correct me if i am wrong .. So trusting ..marking as EF will do interface queuing and at the WAN links we can do shaping / policing with the bandwidth / priority commands in the class map.

Regards,

Gan

[Mark Malone]

Best way to check its working is wirehsark and span the ports make sure its being marked correctly at packet wire but for cli you can use this command among others see docs below

sh mls qos interface gx/x statistics

Correct me if i am wrong .. So trusting ..marking as EF will do interface queuing and at the WAN links we can do shaping / policing with the bandwidth / priority commands in the class map.

There is 2 different types srr shaped round robin and wrr weighted round robin , this link explains it better than I can , you will see what your using in global config when mls global is applied if its 3560 I think srr, there is a priority queue where voice gets serviced first  ---http://blog.ine.com/2008/03/03/bridging-the-gap-between-3550-and-3560-qos-part-i/

I would read this as well it explains everything about MLS and touches on the class maps as well its very well done , there's quite a bit to know before deploying it

http://www.davidsudjiman.info/2012/02/14/cisco-3560-mls-qos-part-1/