Priority Mapping Overview - H3C S5120-EI Series Operation Manual

The precedence is called 802.1p priority because the related applications of this precedence are
defined in detail in the 802.1p specifications.

Priority Mapping Overview

When a packet reaches a switch, the switch assigns the packet parameters according to it configuration,
such as 802.1p precedence, DSCP precedence, IP precedence, local precedence, and drop
precedence.
The local precedence and drop precedence are described as follows.
Local precedence is the precedence that the switch assigns to a packet and it is corresponding to
the number of an outbound queue on the port. Local precedence takes effect only on the local
switch.
Drop precedence is a parameter that is referred to when dropping packets. The higher the drop
precedence, the more likely a packet is dropped.
Depending on whether a received packet is 802.1q-tagged, the switch marks it with priority as follows:
1) For an 802.1q-untagged packet
When a packet carrying no 802.1q tag reaches a port, the switch uses the port priority as the 802.1p
precedence value of the received packet, searches for the local precedence value corresponding to the
port priority of the receiving port in the 802.1p-precedence-to-local-precedence mapping table, assigns
the local precedence value to the packet, and enqueues the packet according to the local precedence
value.
2) For an 802.1q-tagged packet
When an 802.1q tagged packet reaches the port of a switch, you can specify a priority trust mode for the
port, trusting port priority or trusting packet priority.
Trusting packet priority
In this mode, the switch searches for the set of precedence values corresponding to the trusted type
(802.1p precedence or DSCP precedence) of priority of the packet in the corresponding priority
mapping tables and assigns the set of matching precedence values to the packet.
Trusting port priority
In this mode, the switch replaces the 802.1p priority of the received packet with the port priority,
searches for the local precedence corresponding to the port priority of the receiving port in the
802.1p-to-local precedence mapping table, assigns the local precedence to the packet, and enqueues
the packet according to the local precedence value.
You can configure the priority trust mode of a port as required. The priority mapping process on a switch
is as shown in Figure 3-4.
3-4