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 the switch’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-overridefor 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 15-18on 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

queue

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.

IP-precedence

The upper three bits in the Type of Service (ToS) field of an IP packet.

bits

 

upstream

A device linked directly or indirectly to an inbound switch port. That is, the switch receives traffic from

device

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

VLAN

environment, IP packets carry an 802.1p priority from one device to the next.

 

 

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