Configuring 802.1p-based Prioritization and Marking on Egress Interfaces

In Layer 3 networks, traffic priority is typically indicated in the Type of Service (ToS) or Diff Serv Code Point (DSCP) fields in the IPv4 or IPv6 headers of the packets.

Switches can be programmed to map these Quality of Service (QoS) or Class of Service (CoS) values to one of 8 traffic classes and then to use such class assignment to place packets into one of 8 egress queues of a port. This important capability (also known as class-based traffic queuing) helps to deal with traffic congestion in the network’s choke-points.

However, in certain network designs such as, multi-tenant DC designs, it may not be always possible to use the ToS or DSCP fields for class-based prioritization.

An alternative solution is offered by the IEEE 802.1p standard classification, which allows any VLAN-tagged Ethernet traffic to be explicitly classified based on a Class of Service (CoS) 3-bit field, (also known as Priority Code Point (PCP).

Therefore, since packets forwarded on a Layer 3 interface do not usually carry an IEEE VLAN tag (and associated CoS field) by default, this enhancement introduces the capability of explicitly adding a CoS field in the traffic egressing a vRouter interface.

NetVisor OS now supports a new parameter for the configuration of an IEEE 802.1p priority tag (i.e., CoS) on an interface:

CLI (network-admin@switch) > vrouter-interface-add priority-tag|no-priority-tag


CLI (network-admin@switch) > vrouter-interface-modify priority-tag|no-priority-tag

When priority-tag is selected, NetVisor OS adds a CoS value based on the following ingress scenarios and configuration:


  • Case A: On a vRouter interface when the ingress traffic contains an IEEE 802.1p priority tag (CoS), the default behavior is to trust such classification. Hence NetVisor OS uses the same CoS value received in ingress for the egress traffic.

  • Case B: When the ingress traffic is untagged, NetVisor OS uses the default CoS value (0) for the egress traffic on the vRouter interface.

  • Case C: When the ingress traffic contains a DSCP value and if the network admin adds a DSCP map to the port, then NetVisor OS will set the egress CoS value according to the configured DSCP map. In other words, when this specific configuration is chosen by the network admin, the cases A and B is overridden and the egress traffic’s CoS value will instead be decided by the ingress DSCP map (while the ingress CoS value is ignored).