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Community Member

Qos and the evil data Vlan!

Who has the million dollar answer!

We have a mysterious issue... or mayb not to some :)

Our packets of voice are tagged with voice all the way along our hits the data vlan then its gone! if we have a voice vlan instead on that interface then theres no issues what so ever, browny points to the winner :))




Re: Qos and the evil data Vlan!

on ur access port

u have voice vlan and data vlan ?

Community Member

Re: Qos and the evil data Vlan!

yup, its just a data vlan running onto a router which stretches to our WAN :)


Community Member

Re: Qos and the evil data Vlan!

it seems that the qos gets lost when the ip packet moves into any data vlan.. thats what we have concluded so far.


Re: Qos and the evil data Vlan!

dan u desicrption not details

but i can guess u are losing the qos marking when the vlan get routed or in other word when going from L2 to L3

check the following

u need to have trust cos on the switch port level on the voice vlans

onthe switch uplink ports(port from switch to switch and from switch to router including trunks as well) u need to rust DSCP

this way u will preserve the qos marking

try it and let me know

this is called trust boundaries

if helpful Rate

Community Member

Re: Qos and the evil data Vlan!

We have the trust cos on all the switch port levels and trunks etc. and at the switch level that connects we have trust dscp already... strange.

Re: Qos and the evil data Vlan!

if u can post me a simple topology of ur case with small descrition on it

Re: Qos and the evil data Vlan!

Some clarifying questions:

You say that your voice packets are tagged with voice all the way along the network. How are you verifying this and do you mean both the COS and the DSCP bits?

If I remeber correctly, the IP phone only sets the COS priority (IP Precedence or 802.1p). It is left to the first hop L3 device to translate that IP Precedence to a DSCP setting. And to set the IP Precedence in the frame on the next subnet, as the original frame (the one sent from the IP phone) is lost at the router.

Obviously, if the voice traffic to moving from a voice VLAN to a data VLAN, it is being routed at L3. Do you have multiple voice VLANs and does the QOS setting survive moving from one voice VLAN to another?

Community Member

Re: Qos and the evil data Vlan!

Hiya sorry for the delay was off on holiday :)

To clarify.. the DSCP is tagged all the way along from the Edge Switch<3650> ---> 4511 ---> 3750 and our wan router is attached to the 3750. the qos/dscp is there all the way until that port. if that port is in a voice vlan.. it arrives fine no change but if its in the data vlan its rewritten back to 0.

I have checked and the qos dies when it hits any data vlan.

Hope this helps :)

Re: Qos and the evil data Vlan!

first i wish u had a nice holyday

ok now i thing the view is getting more clear

can u tell me which commands u use to check the qos along the way

and u mean that port is connected to the router that connected to the 3750?

if this is the case

can u tell me what qos config if on the 3750

like marking, trust, cos/dscp-map on the port base

and u have the intevaln routing on that switch as well ?

thank you

Community Member

Re: Qos and the evil data Vlan!

thank you thank you :)

we just used wireshark along the way in conjunction with the monitor session command.

yes thats correct :)

its all auto mls qos etc, and it has trust DSCP. i also turned off the auto rewrite mls qos but still to no avail.

yes inter :)


Re: Qos and the evil data Vlan!

i know this is not the ideal way bit try

to make PBR

policy based routing apply it on the data vlan interface i mean the SVI vlan interface

that match any traffic sourced from voice vlan going to any

and set the action to mak the dscp to EF

access-list 100 permit udp [voice vlan IP ]any range 16384 32767

class-map voice

match access-group 100

policy-map voice

class voice

set ip dscp EF

and apply it in the inbound direction on the SVI

and let me know

Re: Qos and the evil data Vlan!

OR if u want better way

make the switch port as truk and trust DSCP

on the router make it trunk and make two subinterfaces one in voice vlan and the other in data vlan


make the link fron ur switch to the router as routed port

in this way u will route the traffic as it passing a router interface from ur switch

becuase logicaly u dont need to take the voice traffic over data in the same vlan

becuase the idea is to separate them logicaly

in ur network along way till the exit point

and as u see at that point u have the problem

good luck

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