mls qos on 3560 switch

Answered Question
Aug 8th, 2012
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Hi All,


We have just enabled "mls qos" and executed default mls qos values.


Can you please explain me in simple words ,what it does ?


Thanks

Sri

Correct Answer by John Blakley about 5 years 1 week ago

COS stands for "class of service" and is l2 only. COS values are from 0 - 7 with 0 being low priority and 7 being high priority. Generally, phones will mark with cos 5 or a dscp value of 46 (EF). There is a cos-dscp mapping that happens outbound on the switch when it sends to the router for the router to see the dscp value that was converted from the internal map on the switch. Once your traffic is tagged at ingress (on switchport), the tag is carried to the output queue on the switch which leads to the router.


Here's something to read:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_55_se/configuration/guide/swqos.html


QoS is a very big topic...

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John Blakley Wed, 08/08/2012 - 04:14
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It enables qos globally on the switch across all ports. By default, the switch will mark everything with cos 0.


HTH,

John

s.kanth Wed, 08/08/2012 - 04:17
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Thanks john for your quicky reply..


COS is L2 QOS, what is L2 QOS.. please explain me in a short ?


Thanks

Sri

Correct Answer
John Blakley Wed, 08/08/2012 - 04:23
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COS stands for "class of service" and is l2 only. COS values are from 0 - 7 with 0 being low priority and 7 being high priority. Generally, phones will mark with cos 5 or a dscp value of 46 (EF). There is a cos-dscp mapping that happens outbound on the switch when it sends to the router for the router to see the dscp value that was converted from the internal map on the switch. Once your traffic is tagged at ingress (on switchport), the tag is carried to the output queue on the switch which leads to the router.


Here's something to read:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_55_se/configuration/guide/swqos.html


QoS is a very big topic...

s.kanth Wed, 08/08/2012 - 04:32
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Hi John,


Thanks for your info.. Got it what i am looking for :-)

Joseph W. Doherty Wed, 08/08/2012 - 05:29
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Just want to mention . . .


L2 CoS requires VLAN tags to provide the CoS field between devices.


L3 ToS is part of every IP packet header.  DSCP is the current recommendation for using the first 6 bits of the ToS byte.


Some "modern" L2 intelligent/smart switches support examination of L3 ToS.


The term CoS might also be use generically or for internal switch QoS treatment (on some Cisco switches).

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