auto qos help

[carl_townshend]

Hi all

I enabled auto qos on my 2960 switches, it has put mls qos trust cos, which is fine as it will map to dscp anyway, but the uplinks from my 2960 to my core also have trust cos, the core switch in the datacentre has trust dscp configured to the 2960's

what should I be using on the uplinks??

also if its configured to trust cos, would the switch write the dscp value in the Ip header??

cheers

[Shashank Singh]

Hi Carl,

First of all mls qos trust *** command affects only ingress traffic. So mls qos trust *** on the uplink will NOT affect traffic which is going to the core. Instead it will only affect traffic coming from the core...just a clarification..

Secondly, if mls qos trust dscp is being used on the core switch, it will be better if you trust dscp on the 2960 uplink also. The reason being the fact that if you trust cos on the 2960 uplink, traffic coming with dscp marking to this switch may not be accurately prioritized.

Moreover if any untagged traffic comes from the core to the 2960, trusting cos does not make any sense (as cos is a part of dot1q tag).

Trusting cos on the access ports connected to phones (due to auto qos) does not make any difference as it anyways gets converted to an internal dscp by the switch for applying QoS.

Cheers,

Shashank

P.S. Please rate the helpful post

[carl_townshend]

hi there

so are you saying its ok to leave trust cos on the access ports, does the switch rewrite dscp of the packet if it only sees the cos value, ?

I will leave cos on the access and then dscp on the uplinks.

cheers

[Shashank Singh]

Hi,

No it is not OK to trust cos on all access ports. Access ports do not receive tagged packets (and hence no cos is there), but when a phone is connected on an access port (thats when we use auto qos voip trust on access port), it sends out dot1p tagged packets on the access ports which contain cos value, and hence trusting cos works.

I would suggest that we trust dscp on all the access ports which are not configured with auto qos voip trust.

Cheers,

Shashank

P.S. Please rate the helpful post.

[carl_townshend]

Hi there

my switches are configured with

autoqos voip trust on all access ports with phones plugged in

and trust dscp on the uplinks to the core

Is this OK??

Also, if i trusted cos, would the switch write equivalent the dscp on egress ??

[Shashank Singh · Accepted solution]

Hi Carl,

Yes this is absolutely OK.

If you trust cos and ingress is trunk port, switch will use it to apply OoS but will NOT rewrite the packet with equivalent dscp on egress.

Cheers,

Shashank

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