DCB QoS Classification

The Linux dcb tool from iproute2 provides the standard upstream interface for configuring Data Center Bridging QoS classification. It controls how ingress frames are mapped to internal priority queues based on PCP (802.1p), DSCP, or a default priority.

This is the upstream-compatible alternative to the proprietary qos tool documented on the Basic QoS page.

1. Platform Support

Feature Sparx5 LAN966x LAN969x LAN9645x

dcb apptrust

✔

✔

✔

✔

dcb app (default-prio, pcp-prio, dscp-prio)

✔

✔

✔

✔

dcb rewr (egress rewriting)

✔

✔

✔

·

dcb pfc (Priority Flow Control)

·

·

·

✔

2. Limitations Compared to the qos Tool

The dcb tool uses the standard Linux DCB framework which is limited to 8 internal priorities (0-7). The hardware supports 16 traffic classes (8 priorities × 2 drop precedence levels). This means:

  • The dcb PCP-to-priority mapping is lossy: frames with different DEI values can be mapped to the same internal priority, but the hardware drop precedence distinction is lost. The qos tool can map PCP+DEI to the full 16 traffic classes.

  • Egress PCP/DEI rewriting via dcb rewr supports mapping internal priority to PCP, but cannot independently control DEI based on drop precedence. The qos tool supports additional egress rewriting modes (e.g., classified PCP/DEI, mapped, port default) that are not available through dcb.

For applications that require full DPL-aware classification or advanced egress rewriting, use the qos tool instead.

3. App Trust

The dcb apptrust command controls which packet fields are trusted for priority classification on ingress. By default, no fields are trusted and all frames use the port default priority.

Trust PCP (VLAN priority) on a port:

$ dcb apptrust set dev eth0 order pcp

Trust DSCP (IP header) on a port:

$ dcb apptrust set dev eth0 order dscp

Disable all trust (use default priority only):

$ dcb apptrust set dev eth0 order

4. Default Priority

The default priority applies to all frames that do not match a trusted field.

Set the default priority to 4:

$ dcb app replace dev eth0 default-prio 4

Reset to default (priority 0):

$ dcb app replace dev eth0 default-prio 0

Show the current default priority:

$ dcb -j app show dev eth0 default-prio

5. PCP-to-Priority Mapping

When PCP trust is enabled, the pcp-prio mapping controls how the 802.1p PCP and DEI fields are mapped to internal priorities.

The mapping format uses <PCP><DEI_FLAG>:<PRIORITY> where:

  • nd = no DEI (DEI=0)

  • de = DEI set (DEI=1)

Configure a custom PCP-to-priority mapping (reverse order for DEI=0):

$ dcb app replace dev eth0 pcp-prio \
  0nd:7 1nd:6 2nd:5 3nd:4 4nd:3 5nd:2 6nd:1 7nd:0 \
  0de:0 1de:1 2de:2 3de:3 4de:4 5de:5 6de:6 7de:7

Restore the identity mapping (default):

$ dcb app replace dev eth0 pcp-prio \
  0nd:0 1nd:1 2nd:2 3nd:3 4nd:4 5nd:5 6nd:6 7nd:7 \
  0de:0 1de:1 2de:2 3de:3 4de:4 5de:5 6de:6 7de:7

6. DSCP-to-Priority Mapping

When DSCP trust is enabled, the dscp-prio mapping controls how the 6-bit DSCP field in the IP header is mapped to internal priorities.

Map DSCP 32 to priority 4:

$ dcb app add dev eth0 dscp-prio 32:4

Remove the mapping:

$ dcb app del dev eth0 dscp-prio 32:4

Without explicit DSCP mappings, all DSCP values map to the default priority.

7. Egress Priority Rewriting (dcb rewr)

Egress rewriting via dcb rewr is supported on Sparx5 and LAN966x only. LAN9645x does not support dcb rewr.

The dcb rewr command controls how the internal priority is mapped back to PCP/DEI values in egress VLAN tags.

Set a priority-to-PCP mapping for egress:

$ dcb rewr set dev eth0 prio-pcp \
  0:0nd 1:1nd 2:2nd 3:3nd 4:4nd 5:5nd 6:6nd 7:7nd

Show the current rewrite mapping:

$ dcb rewr show dev eth0 prio-pcp