trust dscp on 2950

[Naoki7964]

Hi,

i have Catalyst2950SI with iOS12.1,

connect a wifi-access-point  to f1/1(dot1q trunk port),

and connect another L2SW to f1/2(dot1q trunk port),

and IP phone, MobileCamera connects to wifi-access-point,

IP phone has dscp=40 value on its own packet,

but MobileCamera doesn't  have any dscp value or cos.

now, i wanna do QoS by that dscp, So i type as below,

interface 1/1

switchport mode trunk

mls qos trust dscp

interface 1/2

switchport mode trunk

mls qos trust cos

That's all. but never work.

What's the problem?

im appreciate any suggestions.

Thankyou in advance.

[John Blakley]

You also need to enable qos on the switch....

In global mode "mls qos"

HTH,
John

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[Naoki7964]

Thanks John.

but manualbook says that 2950 doesnt have mls qos command.

i dont have tested it though,

[John Blakley]

My apologies, you're correct...here's a post that might help:

https://supportforums.cisco.com/thread/2073790

HTH,
John

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[Naoki7964]

After that, i added below then qos started working.

interface 1/3

switchport access vlan 10

switchport mode access

mls qos cos 3

Dont know why...  Just maybe "mls qos cos" command is the trigger?

[John Blakley]

Hmm...The mls qos cos sets a cos value on a packet if it doesn't have a marking. Are you sure your devices are marking what you think they should be? The mls qos trust would work if your device is marking and you don't want to set it back to the default.

HTH,
John

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[Naoki7964]

Hi, Thanks again, John.

yes, access port hos no dot1q tag, so that has no cos value too.

This access port f1/3 is just a... an example... or is just a bit try to enable QoS function.

Trunk f1/1 and f1/2 is my main target that trusts dscp or trusts cos.

By the way. if i type like below, Whatd happen when packets with dot1q tag but cos value=0 comes?

interface f1/2

switchport mode trunk

mls qos cos 2

mls qos trust cos

C2950 gives a cos=2 to it? or pass through it because it has cos=0 already?

cos=0 is differ from it doesnt have cos.?

best regard.

[John Blakley · Accepted solution]

In your example, the mls qos cos 2 allows for the switch to mark a non-marked packet at 2. If it has a marking that it sees come in, it will leave the marking as is. If you want to override the marking that you receive, you'd use "mls qos cos 2 override". So, in your case, if you used this option and you had someone sending a cos of 5, then you could override that marking down to 2 with "mls cos 2 override". If you wanted to keep that marking then you would use the configuration that you have above and it would trust the marking for 5, but if it received anything that wasn't marked it would mark to 2.

HTH,
John

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[Naoki7964]

Override! Thats it!

and non-marked is equal to untagged, right?

Thanks a lot.

P.S.

Still i have no idea of what is the trigger that qos suddenly start working. but its OK.

[John Blakley]

Well, non-marked doesn't equal untagged necessarily. Untagged is a vlan concept in which there's no vlan tag in the packet whereas non-marked just means there's no marking in the 3-bit field that cos uses for marking.

HTH,
John

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[Naoki7964]

oh, yes, right, cos is on tag or isl and dscp is on ipheader, right.