dscp to cos mapping

Unanswered Question
Mar 20th, 2011

Hello QOS Guru,

I am trying to set up qos and would like some clarification on the "cos-to-dscp" and "dscp-to-cos" mapping to ensure my understanding is correct.

Here is my scenario:

cisco phone --- f0/1-(3560sw)-gig0/1-----trunk 802.1q----- gig0/2-(3750sw)-gig0/1------gig01-(Service provider Router).

I understand cisco phone will send both cos5 and dscp 46 markings by default, so on the 3560 sw port f0/1 i have elected to use the " mls qos trust cost " command to trust cos instead of dscp. The "mls qos trust cos" command will now tricker the 3560sw to use the internally mapping cos-to-dscp and this will  ( Show mls qos map gives me cos-dscp 0 8 16 24 32 46 48 56)  convert the cos value to dscp 46 when egress out the 3560 gig0/1 to the uplink 3750sw gig0/2.  The below part  is where i am at a lost ........

On the 3750 gig0/2 ingress port, do i use the mls qos trust dscp at this point? My concern is if i use the "trust dscp" the 3750 sw will convert the marking from dscp to cos due to the internally mapping on the 3750sw and  on egress gig0/1 the service provider will get a cos value instead of a dscp value? or maybe not, since it isn't a trunk link between the 3750sw and the SP router?

I understand the Service Provider would want dscp and not cos value.



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Eugene Lau Sun, 03/20/2011 - 18:36

Hi Lou,

I think what is confusing you is the CoS -> DSCP mapping.

Let's start from the IP phone where it will generate a voice packet which has CoS5/DSCP46. We need to keep in mind that CoS is Layer2 and DSCP is Layer3.

On the 3560- as you trust cos of 5 -> the DSCP will be mapped according to the CoS-DSCP map, which in your case is 5 - 46 - so no changes.The switch does not "convert" CoS to DSCP. The switch may modifiy the fields but CoS value will exist in 802.1q tag (12bit VLAN identifier | 3 bit Prioritization) and a DSCP value will exist in IP header

When the traffic arrives on g0/2 3750 -> it will have CoS5/DSCP46 -> Trust cos -> access port g0/1 DSCP46 queue to SP router (queue according to DSCP)

So the g0/1 where you have an SP router should be trusting DSCP for return traffic



loyoung Mon, 03/21/2011 - 14:19

Hi Eugene,

Really appreciate your quick response to my post.

So, can i say the cos-to-dscp and dscp-to-cos mapping internal to the switch is really for the switch to know where to place the appropriate marking to the respective queue?.

If cos 5/dscp 46 will be seen from the point of entry on the 3560sw to the engress port g0/1 (where the router is connected), it would be best to just trust dscp all the way through, no? .



Eugene Lau Mon, 03/21/2011 - 15:47

Hi Lou,

That is correct. The internal DSCP or CoS is used to evaluate the packet for scheduling and assigns the packet to one of the queues.

Trusting DSCP throughout the network will work however as with everything in engineering a network, it's about practice. I always thought logically, in an L2  network or connected to a L2 802.1q link it's easier to think to trust CoS. Where it's Layer3 or connected to a L3 link - trust DSCP. It can be up to you, as long as it is consistent in the network.

Perhaps some bed time reading for you? Enterprise QoS Design - SRND - Solution Reference Network Design guides





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