6
Quality of Service (QoS): Managing Bandwidth More Effectively
Introduction
Precedence Criteria Overview
Incoming Where a VLAN-tagged packet enters the switch through a port that is a tagged member of that
802.1p VLAN, if QoS is not configured to override the packet’s priority setting, the switch uses the
Priority 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 Outbound Port Exiting
(Inbound) 802.1p Queue (Outbound)
Priority 802.1p Priority
1 - 2 Low 1 - 2
0 - 3 Normal 0 - 3
4 - 5 Medium 4 - 5
6 - 7 High 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.
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