Cisco 9300 QoS - CoS configuration

[ciscoAvi]

I'm hope to setup Cisco with QoS in L2 environment with VoIP traffic. The packets ingress the SW will be marked with DSCP values. The requirement says that if L2 QoS if necessary the L2 Ethernet switches should be used that can implement QoS measures on base of the L3 DSCP field. I want to try L2 QoS, because in time the L2 infrastructure will most likely grow.

With the requirement my first thought is to setup: Map DCSP-COS.

My question is what will be the best practice to do with this marked CoS frames? The problem will occur when the trunks between the SW-es are congested. Are there a queue for VoIP CoS-frame who can be used in the 9300 catalyst SW. 

[Georg Pauwen]

Hello,

by default, with traffic coming in on a trunk and leaving a trunk, both DSCP and CoS will be trusted. So whatever CoS the voice traffic is marked with, will traverse the trunk.

Either way, the easiest way to configure the trunks would obviously be to use 'auto qos voip trust' (or 'auto qos voip cisco-phone', in case you use Cisco IP phones).

[ciscoAvi]

What will happen if a trunk between two SW are congested? Will frame with cos marked as VoIP-traffic be preferred in a queue, so this frame will go first?

[Joseph W. Doherty · Accepted solution]

Generally, on Cisco switches that support IP DSCP, it can be used for QoS even on L2 ports.  I.e. If you have DSCP values, you often don't need to use L2 CoS at all, and if you don't need to, I recommend you don't.

One issue with L2 CoS, it requires a VLAN tag.  A second issue with L2 CoS, it's not as granular as L3's ToS.

[ciscoAvi]

Thanks for the response Joseph! That sounds like a good advice not to use the CoS-value if the switch supports DSCP-Value. We use catalyst 9300, so these device supports DCSP.

Do you have any advice for best practice to setup QoS on VoIP-traffic(SIP) between SW-es, in regard to congestion. Is it possible to setup a Priority queue, so if congestion occur the SW will look at the SIP package DSCP-value and put it in front, so this package will be transmitter first on the trunk to the next SW, and reduce its latency. 

[Joseph W. Doherty]

Yes, generally VoIP bearer traffic (not VoIP control traffic) QoS recommendation is to use PQ (on switches) (and LLQ on routers).  This not only to minimize additional latency but to also minimize jitter.

BTW, this also applies to any port carrying VoIP (bearer) traffic which may congest.  For example, often a VoIP phone and PC will share an access/edge port.  So the forgoing applies to them too.

[ciscoAvi]

Thanks a lot for the response! Now it makes more sense. I appreciate it.

[Seroj Hacopian]

any cisco documentation URL's for configuration on Qos recomendations?

Thanks

[Joseph W. Doherty]

For actual device configuration, there's the device's IOS reference manual and configuration guide.

For more general design documentation, you might start with Cisco's various QoS design guides, links found on the following:

https://www.cisco.com/c/en/us/solutions/enterprise/design-zone-ipv6/design-guide-listing.html