Quality of Service (QoS) in Cisco ACI

Configuring Quality of Service (QoS) in Cisco ACI (Application Centric Infrastructure) involves creating and applying QoS policies that control traffic prioritization across the fabric. Cisco ACI uses QoS classes (Class-Map, Policy-Map) mapped to Traffic Classes (TC) and DSCP values to manage traffic.

Here’s a step-by-step guide to configure QoS in Cisco ACI using the APIC GUI (can also be done via REST API or CLI):

Step-by-Step QoS Configuration in Cisco ACI

1. Create QoS Classes

Define QoS classes that map to different traffic priorities.

Steps:

Go to Tenants > Tenant (e.g., common).
Expand Policies > Protocol > QoS Classes.
Click + QoS Class.
Enter a name (e.g., High-Priority, Medium-Priority, Low-Priority).
Set the Traffic Class (1-7).
Choose Bandwidth if needed (e.g., Min/Max %).
Optionally set Burst and DSCP marking.

2. Create a QoS Policy

This policy defines how traffic should be treated based on matching rules.

Steps:

Go to Tenants > Tenant (e.g., common).
Expand Policies > Protocol > QoS Policies.
Click + QoS Policy.
Provide a name and description.
Select QoS Class and define DSCP marking if applicable.

3. Apply the QoS Policy to Interfaces or EPGs

QoS policies can be applied to:

Interface Policies (Access or VPC)
EPGs (Endpoint Groups)

For Interface:

Go to Fabric > Access Policies > Interface Policies > Policy Groups.
Choose your interface policy group (Leaf Interface or VPC).
Attach the QoS Policy created earlier.

For EPG:

Go to Tenants > YourTenant > Application Profiles > YourApp > EPGs.
Select an EPG.
Under Policy, choose the QoS Policy from the drop-down.

4. (Optional) Use Contracts to Mark QoS

You can configure Contracts to apply QoS marking to specific traffic between EPGs.

In the Contract > Subject > Filters, set DSCP markings for specific traffic types.

Quick Notes

Traffic Class (TC) values map directly to internal priority queues in the fabric.
You can map DSCP to TC using the Global QoS Policy under System > System Settings > QoS.
Be mindful of tenant scope — QoS policies can be tenant-specific or in the common tenant to share across.

Best Practices

Keep critical traffic (voice, control) in higher-priority classes (e.g., TC 5-7).
Avoid overusing the highest priority class to prevent starvation of other traffic.
Use DSCP-based marking for better interoperability with external networks.

Scenario: Prioritize VoIP Traffic Over Web and Bulk Traffic

Goals:

VoIP traffic gets highest priority (low latency, low jitter).
Web traffic (HTTP/HTTPS) gets medium priority.
Bulk traffic (like backups) gets low priority.

Step-by-Step Configuration

1. Define QoS Classes

Go to:Tenants > common > Policies > Protocol > QoS Classes > + QoS Class

Create the following classes:

Name	Traffic Class	DSCP Marking	Bandwidth %
VoIP-Class	5	EF (46)	30%
Web-Class	3	AF21 (18)	40%
Bulk-Class	1	CS1 (8)	30%

2. Create QoS Policy

Go to:Tenants > common > Policies > Protocol > QoS Policies > + QoS Policy

Create three policies and map them to the above classes:

QoS Policy Name	QoS Class
VoIP-QoS	VoIP-Class
Web-QoS	Web-Class
Bulk-QoS	Bulk-Class

3. Apply QoS Policies to EPGs

Let’s say you have three EPGs:

EPG Name	Traffic Type
VoIP-EPG	VoIP
Web-EPG	Web
Backup-EPG	Bulk

Go to:Tenant > YourAppProfile > EPGs > EPG > Policies > QoS Policy

Assign:

VoIP-EPG → VoIP-QoS
Web-EPG → Web-QoS
Backup-EPG → Bulk-QoS

4. Optional: Mark QoS via Contracts

If traffic is flowing between EPGs via contracts, you can apply DSCP markings.

Go to:Tenant > Contracts > Subject > Filters > +Filter Entry

Create filter entries like:

Filter Name	Protocol	Ports	DSCP
VoIP-UDP	UDP	16384-32767	EF (46)
Web-TCP	TCP	80, 443	AF21 (18)
Bulk-TCP	TCP	445, etc.	CS1 (8)

Use APIC GUI or CLI to check:

show qos-class-map

show policy-map interface

Also use EPG Monitoring > Traffic > QoS Stats to validate traffic classes.

For VoIP, always use Expedited Forwarding (EF / DSCP 46) and make sure your upstream/downstream switches honor that marking (end-to-end QoS).