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

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 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-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 6-43.For the DSCP map, see figure 6-19on page 6-44.

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 classifiers 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

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

bits

 

IPv4

Version 4 of the IP protocol.

IPv6

Version 6 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.

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