About 802.1p Marking for VLAN Interfaces

802.1p is a quality of service (QoS)/class of service (CoS) method that operates at the MAC layer (Layer 2). Equipment that supports 802.1p can add and recognize a value that indicates the priority level of the Ethernet frame. 802.1p can help to ensure a high level of quality for real-time communications that are sensitive to latency, such as VoIP.

The Firebox and other 802.1p-enabled equipment prioritize traffic based on the priority value. 802.1p applies to only VLAN member interfaces that are configured to send and receive VLAN-tagged traffic. This includes outbound traffic that originates from the Firebox and also outbound traffic that travels through the Firebox.

When you enable 802.1p for a VLAN interface, the Firebox marks outbound traffic from that interface. The Firebox adds an 802.1q tag to Layer 2 Ethernet frames and copies the IP precedence value from the Layer 3 IP header to the priority code point (PCP) 802.1p field in 802.1q tag.

How It Works

This diagram shows 802.1q and 802.1p information in Layer 2 ethernet frames.

Screen shot of 802.1q and 802.1p at Layer 2

The Differentiated Services Code Point (DSCP) value in IP packets maps to the PCP value:

  • IPv6 packets — Upper three bits of the DSCP value in the Traffic Class (TC) field maps to the PCP
  • IPv4 packets — Upper three bits of the DSCP value in the Type of Service (ToS) field maps to the PCP

The DSCP value comprises six bits (0–63). This table shows how DSCP values map to 802.1p PCP values.

DSCP Value Range (Layer 3 Packet) 802.1p PCP Priority Value (Layer 2 Ethernet Frame)
0–7 0 Best effort
8–15 1 Background
16–23 2 Spare
24–31 3 Excellent effort
32–39 4 Controlled load
40–47 5 Video (<100 ms latency)
48–55 6 Voice (<10 ms latency)
56–63 7 Network control


To mark outbound VoIP traffic on a VLAN interface with a PCP value of 6, the Firebox copies the DSCP value from the packet to the three-bit PCP 802.1p field in 802.1q tag of the frame.

  • The DSCP of the ToS field (IPv4) or TC field (IPv6) field is 48. In binary, the number 110000 represents the number 48.
  • The Firebox uses the upper three bits (110) of the DSCP value as the PCP value.
  • The binary number 110 represents the number 6, which means the PCP value is 6.

Diagram of a VoIP example

Enable 802.1p Marking for VLAN Interfaces

To enable 802.1p marking, see Define a New VLAN.

See Also

About Virtual Local Area Networks (VLANs)

Define a New VLAN