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Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Table 15-4. Precedence Criteria for QoS Classifiers
Precedence Criteria Overview
1 UDP/TCP Takes precedence based on a layer 4 UDP or TCP application, with a user-specified application
port number (for example, Telnet). Default state: Disabled
If a packet does not meet the criteria for UDP/TCP priority, then precedence defaults to the Device Priority
classifier, below.
2 Device
Priority
(IP
Address)
Takes precedence based on an outbound packet having a particular destination or source IP
address. QoS allows up to 256 IP addresses. If an outbound packet has an IP address as the
destination, it takes precedence over another outbound packet that has the same IP address
as a source. (This can occur, for example, on an outbound port in a switch mesh environment.)
Also, if the source and destination IP addresses (SA and DA) in the same packet match, the
DA takes precedence. Default state: No IP address prioritization.
If a packet does not meet the criteria for device priority, then precedence defaults to the IP Type of Service
(ToS) classifier, below.
3 IP Type-
of-
Service
(ToS)
Takes precedence based on the TOS field in IP packets. (Applies only to IP packets.) The ToS
field is configured by an upstream device or application before the packet enters the switch.
• IP Precedence Mode: QoS reads the packet’s IP precedence (upper three) bits in the Type-
of-Service (ToS) byte and automatically prioritizes the packet (if specified in the QoS
configuration) for outbound transmission.
• Differentiated Services Mode: QoS reads the packet’s differentiated services, or codepoint
(upper six) bits of the Type-of-Service (TOS) byte. Packet prioritization depends on the
configured priority for the codepoint. (Some codepoints default to the DSCP standard, but
can be overridden.)
For more on this topic, see “QoS IP Type-of-Service (ToS) Policy and Priority” on page 15-25.
Default state: Disabled.
If a packet does not meet the criteria for ToS priority, then precedence defaults to the Protocol classifier,
below.
4 Layer 3
Protocol
Priority
Takes precedence based on network protocols: IP, IPX, ARP, DEC LAT, AppleTalk, SNA, and
NetBeui. Default state: No-override for any protocol.
If a packet does not meet the criteria for Protocol priority, then precedence defaults to the VLAN classifier,
below.
5 VLAN
Priority
Takes precedence based on the ID number of the VLAN in which the packet exists. For example,
if the default VLAN (VID = 1) and the “Blue” VLAN (with a VID of 20) are both assigned to a port,
and Blue VLAN traffic is more important, you can configure QoS to give Blue VLAN traffic a
higher priority than default VLAN traffic. (Priority is applied on the outbound port.) Default state:
No-override.
If a packet does not meet the criteria for VLAN priority, then precedence defaults to the Source-Port
classifier, below.
15-9