Changing (Dante) DSCP QoS on a Cisco SG350

Dante default DSCP QoS values

  • Dante AES67 audio (RTP) packets are marked with a DSCP value of 46.

  • Dante PTP packets are marked with a DSCP value of 56.

AES67 default DSCP QoS values see AES67 standard.

  • AES67 likes audio (RTP) packets with DSCP value 34

  • AES67 likes to have PTP packets with DSCP value 46

Meaning if you mixed Dante and AES67 packets on the same switch - the Dante audio (RTP) packets would have the same que priority as the AES67 PTP packets. This would cause PTP errors on your AES67 network.

The answer is to re-stamp the Dante packets to have a different DSCP values

A Cisco SG350 allows one to change values according to rules and to maintain best practice, we should change the RTP packets to have DSCP 34 & PTP to have 46 - this matches the AES67 standard.

There are a lot of steps…. And they must be done in order (enforced by the switch). There are actually several ways of arriving at the same place and other methods are detailed in this Cisco guide.

If no other device has packets with DSCP 56 (desirably), one can override (globally) this value on the switch to cover the PTP problem…

 

Set QoS Advanced settings as above and open the DSCP Override table:

 

Map 56 to 46. Now your Dante & AES67 PTP packets have the same DSCP value. If 56 is required for something else, this method cannot be used so just create another ACL (see below) for PTP.

 

Create an ACE for the ACL

 

Above is an example RTP ACE (truncated screenshot). There is a source IP of "Any". The destination has wildcards in the 3rd and 4th byte so that any multicast beginning with 239.69 & DSCP 46 will be caught by the rule (Dante must use a fixed 2nd byte so make sure this matches your setup - 69 is default). The protocol is set to "Any" since RTP is not specifically available and we know that any address in the defined range will be RTP anyway.

Now add a class map (very simple - just map the ACL to a class map):

 

Now add the class map to a policy:

 

In the class map table, set the new DSCP value for the relevant packets:

 

Now bind the policy to any port on the switch with a Dante device - set default action to Permit Any: