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New Member

Trusting dscp across vlans

Am I right in thinking that dscp is used in layer 3 and CoS is used in layer 2, and that to trust a CoS value accross vlans using SVIs, you will needs a cos-dscp map to convert to dscp and then a dscp-cos map to convert to back to CoS on the second vlan?

Thanks!

J

4 REPLIES

Re: Trusting dscp across vlans

Hi

Yes your understanding is correct , when it comes to L3 dscp comes into picture and Cos in a complete L2 environment .So in a L2 network depending on the cos value , traffic is prioritised .When the L2 cloud terminates on the L3 vlan , then comes the mapping of your Cos to DSCP and it happens by default cos-dscp and Vice-versa.

We also need to enable trust dscp or cos in the interfaces connecting the L2 network , so that the switch doesnot make the cos value to default 0 rather than to carry the original cos value.

Hope it helps

regards

vanesh k

New Member

Re: Trusting dscp across vlans

Hi Vanesh,

Thanks for your response.

Actually my problem lies with enabling trust on the layer 3 interfaces; when I try to enter any mls qos command on int vlan 1, it comes up with unrecognised command, despite being displayed when I use "?". I seem to only be able to use it on a physical interface. Do you have any ideas why that may be?

It's a c3750 running Version 12.2(25)SEB1

Regards,

J

Re: Trusting dscp across vlans

Hi

Its all the same with any series of switches . The incoming Cos value or DSCP from the other switch needs to be trusted,so that appropriate mappings could take place .So the trust need to be done only in the physical interface ,as here is the traffic that comes with a set cos value and mapping of dscp happens in the L3 vlan interface.

Hope it helps

regards

vanesh k

New Member

Re: Trusting dscp across vlans

Hi,

Many thanks for your response.

Here is my problem:

I have 2 vlans on 1 switch, I want my packets to carry some form of classification between the 2 vlans.

I therefore want to apply trust to a vlan interface to trust what ever's coming from from the other vlan (with the help of dscp-cos mapping), but I am unable to do so as I can only enter the "mls qos trust" command on a physical interface. There are many physical interfaces assigned to each vlan.

How would I go about this, within one switch?

Thank you,

J

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