6-10
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Table 6-5. Precedence Criteria for QoS types
Precedence Criteria Overview
1 IP Type-
of-
Service
(IP 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 an inbound packet’s IP precedence (upper three) bits in
the Type-of-Service (ToS) byte and automatically assigns an 802.1p priority to the packet (if
specified in the QoS configuration) for outbound transmission.
• Differentiated Services (Diffserve) Mode: QoS reads an inbound IP 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 IP ToS, see “QoS IP Type-of-Service (ToS) Policy and Priority” on page 6-16. Default
state: Disabled.
If a packet does not meet the criteria for ToS priority, then precedence defaults to the VLAN type
2 Interface
(Source-
Port)
Takes precedence based on the Interface (that is, the port on which the packet entered the
switch).
If a packet does not meet the criteria for source-port priority, then precedence defaults to Incoming 802.1p
criteria, below
3 Incoming
802.1p
Priority
Where a VLAN-tagged packet enters the switch through a port that is a tagged member of that
VLAN, if QoS is not configured to override the packet’s priority setting, the switch uses the
packet’s existing 802.1p priority (assigned by an upstream device or application) to determine
which inbound and outbound port queue to use. If there is no QoS policy match on the packet,
and it then leaves the switch through a port that is a tagged member of the VLAN, then there
is no change to its 802.1p priority setting. If the packet leaves the switch through a port that is
an untagged member of the VLAN, the 802.1p priority is dropped.
Entering
(Inbound) 802.1p
Priority
Outbound Port
Queue
Exiting
(Outbound)
802.1p Priority
1 - 2
0 - 3
4 - 5
6 - 7
Low
Normal
Medium
High
1 - 2
0 - 3
4 - 5
6 - 7
If a packet does not meet the criteria for Incoming 802.1p priority, then the packet goes to the “normal”
outbound queue of the appropriate port. If the packet entered the switch through a port that is an untagged
member of a VLAN, but exits through a VLAN-tagged port, then an 802.1Q field, including an 802.1p priority,
is added to the packet header. If no QoS policy is configured or applied to the packet, then the 802.1p priority
of 0 (normal) is assigned to the packet for outbound transmission.
