Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Terminology
Term Use in This Document
codepoint Refer to DSCP, below.
downstream A device linked directly or indirectly to an outbound switch port. That is, the switch sends traffic to
device 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 theswitch’s default
QoS configuration, some codepoints are configured with default 802.1p priority settings for Assured-
Forwarding and Expedited Forwarding, while others 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 classifier, 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 15-34. For the DSCP map, see figure 15-18 on page 15-35.
edge switch In the QoS context, this is a switch that receives traffic 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 classifiers
such as TCP/UDP application type, IP-device (address), Protocol (LAN), 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 then honor.
inbound port Any port on the switch through which traffic enters the switch.
IPv4 Version 4 of the IP protocol.
outbound A packet leaving the switch through any LAN port.
packet
outbound port Any port on the switch through which traffic leaves the switch.
outbound port For any port, a buffer that holds outbound traffic until it can leave the switch through that port. There
four outbound queues for each port in the switch: high, medium, normal, and low. Traffic in a port’s high
priority queue leaves the switch before any traffic in the port’s medium priority queue, and so-on.
queue
IP-precedence
bits
The upper three bits in the Type of Service (ToS) field of an IP packet.
upstream
device
A device linked directly or indirectly to an inbound switch port. That is, the switch receives traffic from
upstream devices.
802.1p priority A traffic priority setting carried by packets moving from one device to another in an 802.1Q tagged
VLAN environment. 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 an untagged VLAN, this priority
is dropped, and the packet arrives at the next, downstream device without an 802.1p priority
assignment.
802.1Q tagged A virtual LAN (VLAN) that complies with the 802.1Q standard and is configured as “tagged”. In this
environment, IP packets carry an 802.1p priority from one device to the next.
VLAN
15-5
