Configuring QoS for Switches

Follow

Quality of service (QoS) allows you to prioritize certain traffic flows over others, to ensure latency sensitive applications (like voice and video traffic, for instance) do not suffer, when the network is loaded or congested. The most common ways that QoS is achieved/ honored in L2+ switches is either through the use of the Class of Service (CoS) bits in the Ethernet frames (L2) or through the use of DSCP bits in the IP header (L3) or both. OM S series switches support both, CoS and DSCP, configurations and allow for QoS settings, based on them. OM S series switches have multiple queues per port, with different priorities. The switch port queues can be mapped to CoS and DSCP values through simple configuration settings in Datto Managed Networking and further, there is also flexibility in being able to configure how the switch queues are serviced, for transmission.

Configuring QoS on the S series switches via Datto Managed Networking can be done in 3 easy steps:

  • Configure DSCP to switch queue and CoS to switch queue mappings - (once for all switches in the network)
  • Enable QoS and configure scheduling policy and trust mode (once per per switch)
  • Configure CoS policy for ingress and egress packets (once per port of the switch)

Step 1: Configure DSCP-to-switch queue and CoS-to-switch queue mappings

This can be done from within the Configure > Switches page as follows:

Click on the "show" link next to the "Quality of Service (QoS) mappings" field

You will see two configuration fields under Voice VLAN, one to configure mapping of DSCP values to switch queues and the other to configure mapping of CoS values to switch queues. Click on each of them and configure things as appropriate:

Configure CoS-to-switch-queue mapping by assigning a switch queue to each of the 8 possible CoS values. 8 traffic queues are supported (1-8), with queue labeled 1 having the lowest priority and queue labeled 8 having the highest priority

Configure DSCP-to-switch-queue mapping by assigning a switch queue to each of the 64 possible DSCP values. 8 traffic queues are supported (1-8), with queue labeled 1 having the lowest priority and queue labeled 8 having the highest priority.

Step 2: Enable and configure QoS settings per switch

The next step is to configure QoS settings per switch. For this, head to the Monitor > Switches page, click on the specific switch to bring up its configuration pane and click on the General tab. Once there, click on the toggle button next to the QoS field. You will have the option to choose a specific scheduling method as well as a trust mode.

The scheduling method dictates the policy that the switch will use to pick outbound packets for transmission from the multiple queues that exist at each port. Strict priority will always prioritize queues with a higher priority while Weighted Round Robin (WRR) will weight each queue by priority and then apply a round robin type policy to pick packets for transmission.

Trust mode tells the switch whether to use the 802.1p (CoS) bits (and mapping to queues) for QoS or the DSCP bits, of incoming packets. When CoS+DSCP is configured the switch will use DSCP bits unless the DSCP bits are not set in the packet, in which case, it will use the CoS bits.

Step 3: Configure CoS policy per port

The last things to specify is the policy that the switch would use, to make decisions about packets that come into a port or leave a port, based on whether they have the CoS bits set or not. This can be done from the ports tab, within the switch pane.

The CoS column specifies the CoS value to set to ingress (incoming) packets that do not already have a CoS value set. If there is a CoS value already set it will be used and if it isn't, then the value set in this column will be used. 

The Trust CoS column is used to make decisions about how to queue packets for transmission. If checked, the CoS value is used to place packets in specific queues (of priority) and if unchecked, all packets are sent to the same queue (no QoS). Note that these per port CoS rules only have impact if the QoS settings for the switch (step 2 above) are configured with a trust mode that includes inspecting the CoS bits. If it is only set to DSCP, then these settings don't matter.


Was this article helpful?

0 out of 0 found this helpful

You must sign in before voting on this article.

Want to talk about it? Head on over to our Community Forum!