AutoQos

[dennisv99]

Dear all,

In my struggle to understand some QoS i did some study on a existing Qos configuration with AutoQos on a C4015R.

A Nortel voip phone is attached to the switch and a pc is attached to the phone.
The switch interface where the phone is connected to is configured as access vlan 107 and voice vlan 104. So if i'm correct the pc data is untagged and the voice traffic is tagged with a 802.1q tag with vlan 104.

AutoQoS is enabled on the switchport with the command auto qos voip trust

A Nortel PBX is connected to the same switch in vlan 200.

The Cisco AutoQoS White Paper states that the dscp values in ingress voip traffic wil be marked as follows:
-Voip bearer traffic with dscp EF (46)
-Voip signaling traffic with dscp AF31 (26)
-Non voip traffic with dscp values EF or AF31 will be remarked with dscp value 0

The Nortel documentation states the following:
"By default, Nortel’s IP phones will mark traffic with 802.1p bit set to 6 and the DiffServ value set to
Explicit Forwarding (EF) 46 for Voice Traffic and 40 for Signalling Traffic."
If i understand correct the mentioned 802.1p bits are the Cos value in the 802.1q header of the voice vlan traffic? Is this true?

The Cos-DSCP mappings on the switch are as follows:
Switch#sh qos maps cos
CoS-DSCP Mapping Table
   CoS:   0  1  2  3  4  5  6  7
--------------------------------
  DSCP:   0  8 16 26 32 46 48 56

When i trace the packets arriving at the pbx they have dscp value of 48. This is voip bearer traffic i think.
Because of this i assume the Cos value is trusted and the dscp values are rewritten bij the mapped values. Is this assumption right?

-How does autoqos write the dscp value to the right packets? In other words how does it recognise voip packets?
-Why aren't the DSCP values (re)written by the value of 46 as the autoqos configuration should do?

[Jon Marshall]

DennisV99 wrote:
Dear all,
In my struggle to understand some QoS i did some study on a existing Qos configuration with AutoQos on a C4015R.
A Nortel voip phone is attached to the switch and a pc is attached to the phone.
The switch interface where the phone is connected to is configured as access vlan 107 and voice vlan 104. So if i'm correct the pc data is untagged and the voice traffic is tagged with a 802.1q tag with vlan 104.
AutoQoS is enabled on the switchport with the command auto qos voip trust
A Nortel PBX is connected to the same switch in vlan 200.
The Cisco AutoQoS White Paper states that the dscp values in ingress voip traffic wil be marked as follows: 
-Voip bearer traffic with dscp EF (46)
-Voip signaling traffic with dscp AF31 (26)
-Non voip traffic with dscp values EF or AF31 will be remarked with dscp value 0
The Nortel documentation states the following:
"By default, Nortel’s IP phones will mark traffic with 802.1p bit set to 6 and the DiffServ value set to 
Explicit Forwarding (EF) 46 for Voice Traffic and 40 for Signalling Traffic."
If i understand correct the mentioned 802.1p bits are the Cos value in the 802.1q header of the voice vlan traffic? Is this true?
The Cos-DSCP mappings on the switch are as follows:
Switch#sh qos maps cos 
CoS-DSCP Mapping Table
   CoS:   0  1  2  3  4  5  6  7 
--------------------------------
  DSCP:   0  8 16 26 32 46 48 56
When i trace the packets arriving at the pbx they have dscp value of 48. This is voip bearer traffic i think.
Because of this i assume the Cos value is trusted and the dscp values are rewritten bij the mapped values. Is this assumption right?
-How does autoqos write the dscp value to the right packets? In other words how does it recognise voip packets?
-Why aren't the DSCP values (re)written by the value of 46 as the autoqos configuration should do?

Dennis

It depends on what your port setting is as much as anything else. If you tell the port to trust CoS then you will indeed end up with DSCP markings of 48 if the CoS value is 6. Nortel phones use a different CoS value. If you want to mark the traffic with the correct value of 46 for Cisco markings then you can modify either cos-dscp map so CoS 6 = 46 or you can from memory modify the DSCP mutation map as well.

Either way you need to modify a map or change the setting on the phone to CoS 5 if that is possible on the Nortel phone.

Jon