QoS examples and Best Practices

The sections that follow present QoS network scenarios for bridged and routed traffic over the core network.

Bridged traffic

If you bridge traffic over the core network, you keep customer VLANs separate (similar to a Virtual Private Network). Normally, a service provider implements VLAN bridging (Layer 2) and no routing. In this case, the 802.1p-bit marking determines the QoS level assigned to each packet. If DiffServ is active on core ports, the level of service received is based on the highest of the DiffServ or 802.1p settings.

The following cases provide sample QoS design guidelines you can use to provide and maintain high service quality in a network.

If you configure a core port, you assume that, for all incoming traffic, the QoS value is properly marked. All core switch ports simply read and forward packets; they are not re-marked or reclassified. All initial QoS markings are performed at the customer device or on the edge devices.

The following figure illustrates the actions performed on three different bridged traffic flows (that is VoIP, video conference, and email) at access and core ports throughout the network.

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Trusted bridged traffic

For bridged, untrusted traffic, if you configure the port to access, mark and prioritize traffic on the access node using global filters. Reclassify the traffic to ensure it complies with the class of service specified in the SLA.

For Resilient Packet Ring (RPR) interworking, you can assume that, for all incoming traffic, the QoS configuration is properly marked by the access nodes. The core switch ports, configured as core or trunk ports, perform the RPR interworking. These ports preserve the DSCP marking and re-mark the 802.1p bit to match the 802.1p bit of the RPR. The following figure shows the actions performed on three different traffic flows (VoIP, video conference, and email) over an RPR core network.

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RPR QoS internetworking

Routed traffic

If you route traffic over the core network, VLANs are not kept separate.

If you configure the port to core, you assume that, for all incoming traffic, the QoS configuration is properly marked. All core switch ports simply read and forward packets. The switch does not re-mark or classify the packets. The customer device or the edge devices perform all initial QoS markings.

The following figure shows the actions performed on three different routed traffic flows (that is VoIP, video conference, and email) at access and core ports throughout the network.

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Trusted routed traffic

For routed, untrusted traffic, in an access node, packets that enter through a tagged or untagged access port exit through a tagged or untagged core port.