6-5
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Terminology
Term Use in This Document
802.1p priority A traffic priority setting carried by a VLAN-tagged packet moving from one device to another through
ports that are tagged members of the VLAN to which the packet belongs. This setting can be from 0 -
7. The switch handles an outbound packet on the basis of its 802.1p priority. However, if the packet
leaves the switch through a VLAN on which the port is an untagged member, this priority is dropped,
and the packet arrives at the next, downstream device without an 802.1p priority assignment.
802.1Q field A four-byte field that is present in the header of Ethernet packets entering or leaving the switch through
a port that is a tagged member of a VLAN. This field includes an 802.1p priority setting, a VLAN tag, or
ID number (VID), and other data. A packet entering or leaving the switch through a port that is an
untagged member of the outbound VLAN does not have this field in its header and thus does not carry
a VID or an 802.1p priority. See also “802.1p priority”.
codepoint Refer to DSCP, below.
downstream
device
A device linked directly or indirectly to an outbound switch port. That is, the switch sends traffic to
downstream devices.
DSCP Differentiated Services Codepoint. (Also termed codepoint.) A DSCP is comprised of the upper six bits
of the ToS (Type-of-Service) byte in IP packets. There are 64 possible codepoints. In the default QoS
configuration for the switches covered in this chapter, one codepoint (101110) is set for Expedited
Forwarding. All other codepoints are unused (and listed with No-override for a priority).
DSCP policy A DSCP configured with a specific 802.1p priority (0- 7). (Default: No-override). Using a DSCP policy,
you can configure the switch to assign priority to IP packets. That is, for an IP packet identified by the
specified QoS type, you can assign a new DSCP and an 802.1p priority (0-7). For more on DSCP, refer
to “Details of QoS IP Type-of-Service” on page 6-26. For the DSCP map, see figure 6-8 on page 6-27.
edge switch In the QoS context, this is a switch that receives traffic from the edge of the LAN or from outside the
LAN and forwards it to devices within the LAN. Typically, an edge switch is used with QoS to recognize
packets based on QoS types such as TCP/UDP application type, IP-device (address), VLAN-ID (VID),
and Source-Port (although it can also be used to recognize packets on the basis of ToS bits). Using this
packet recognition, the edge switch can be used to set 802.1p priorities or DSCP policies that
downstream devices will honor.
inbound port Any port on the switch through which traffic enters the switch.
IP Options In an IPv4 packet, these are optional, extra fields in the packet header.
IP-precedence
bits
The upper three bits in the Type of Service (ToS) field of an IP packet.
IPv4 Version 4 of the IP protocol.
IPv6 Version 6 of the IP protocol.
outbound
packet
A packet leaving the switch through any LAN port.
outbound port Any port on the switch through which traffic leaves the switch.
