User guide

Layer 2 switch functional description Switching
2-5
iMG/RG Software Reference Manual (Switching)
IEEE 802.1p is a spin-off of the IEEE 802.1q (VLAN tagging) standard and they work in tandem (see Figure 1).
The 802.1q standard specifies a VLAN tag that appends to a MAC frame. The VLAN tag carries VLAN informa-
tion. The VLAN tag has two parts: The VLAN ID (12-bit) and User Priority (3-bit). The User Priority field was
never defined in the VLAN standard. The 802.1q implementation defines this prioritizing field.
Switches, routers, servers, even desktop systems, can set these priority bits in the three-bit user priority field,
which allows packets to be grouped into various traffic classes. If a packet is received that does not have this tag
added, then the switch adds it to the packet and uses the default priority associated with the port.
In the two queue systems, the user priority field in the TAG header is compared with an internal value inthe
switch called the base priority - and all values equal or greater to this base priority are put into the high priority
egress queue - while all others are put into the low priority queue.
In the four queue systems, the value in the user priority is used to determine which queue to place the packet
into directly. This mapping is configurable.
2.2.2.7.2 Differentiated services code point (DSCP)
The IEEE 802.1p signalling technique is an IEEE endorsed specification for prioritizing network traffic.
The DSCP octet in the IP header classifies the packet service level. The DSCP replaces the ToS Octet in the
IPv4 header (see Figure 2-1).
Currently, only the first six bits are used. Two bits of the DSCP are reserved for future definitions. This allows
up to 64 different classifications for service levels.
In the two queue systems, the DSCP field is compared with an internal value in the switch called the base prior-
ity - and all values equal or greater to this base priority are put into the high priority egress queue - while all
others are put into the low priority queue.
In the four queue systems, the value in the user priority used to determine which queue to place the packet into
directly. This mapping is configurable.