map QOS to COS-DSCP ?

[jonesm111]

On our Access switches we have 2 different qos to cos-dscp settings;

mls qos map cos-dscp 0 8 16 24 32 46 48 56
mls qos map cos-dscp 0 8 16 24 32 40 46 48

Can someone help me break it down and figure out exactly what this means?

Thanks!

[Jon Marshall · Accepted solution]

jonesm111 wrote:
On our Access switches we have 2 different qos to cos-dscp settings;

mls qos map cos-dscp 0 8 16 24 32 46 48 56
mls qos map cos-dscp 0 8 16 24 32 40 46 48
Can someone help me break it down and figure out exactly what this means?
Thanks!

CoS is a L2 marking contained within an 802.1q tag,. The values for CoS are 0 - 7

DSCP is a  L3 marking and has values 0 - 63

Switches use an internal DSCP value when moving traffic through the switch. However if the marking received is a CoS marking the switch then needs to derive a DSCP value from this value which is what the QOS map cos-dscp is used for.

Obviously the 8 values of CoS cannot exactly map to 64 DCSP values so that's what the above map shows ie.

CoS value 0 = DCSP 0

CoS value 1 = DCSP 8

CoS value 2 = DSCP 16

etc..

One more thing to note CoS value 5 is commonly used for VOIP and CoS value 5 should be mapped to DSCP 46. DSCP 46 is the EF value (Expedited forwarding) which gets priority over the DSCP AF classes.

Jon

[jonesm111]

Thanks Jon,

So would mls qos map cos-dscp 0 8 16 24 32 46 48 56 map COS3 to 24 and COS5 to 46?

We use COS3 for setup, COS5 for payload

[Jon Marshall]

jonesm111 wrote:
Thanks Jon,
So would mls qos map cos-dscp 0 8 16 24 32 46 48 56 map COS3 to 24 and COS5 to 46?
We use COS3 for setup, COS5 for payload

Yes, CoS 3 = 24 and CoS 5 = 46 from the above map.

Jon

[fishtacos]

Great feed. One more question, why are there 2 different rows? The second row mapping is slightly different but is named the same? Thanks!

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

mls qos map cos-dscp 0 8 16 24 32 40 46 48

[Mark Malone]

This thread is 5 years old but to answer your question anyway the 1st line would be created automatically if you enable mls qos globally , the 2nd line was most likely added manually , we had to do the same for some of our switches that connect to our 3rd party wireless controllers that have slightly different dscp settings , instead of cs6 there using cs5  , see the chart link below will help

http://www.netcontractor.pl/blog/wp-content/uploads/2010/06/QoS-Values-Calculator-v21.jpg

[jonesm111]

Glad I was able to help out by starting this thread, interesting when I'm still getting notified on this 5 years later, lol

[janezh]

Very clear explainaton and helpful!

[Joseph W. Doherty]

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Posting

". . . EF value (Expedited forwarding) which gets priority over the DSCP AF classes."

Since Janezh's post "freshen" this thread, I just wanted to note tagging a packet with DSCP EF, or any other DSCP value, alone, does not actually guarantee priority relative to any other DSCP tag.  However, if packets are being QoS treated, based on their DSCP markings, normally DSCP EF would be treated better than BE or other AF classes (as Jon notes).  Preference packet treatment depends on actual QoS policies.  (NB: QoS policies might treat packets differently without even using L3 ToS.)

[alexchen88]

clear and helpful!