QoS operates in VLAN-tagged and VLAN-untagged environments. If your network does not use multiple VLANs, you can still implement the 802.1Q VLAN capability for packets to carry their 802.1p priority to the next downstream device. To do so, configure ports as VLAN-tagged members on the links between switches and routers in your network infrastructure.
Summary of QoS capabilities
| Outbound packet options | Port membership in VLANs | ||
|---|---|---|---|
| Tagged | Untagged | ||
| Control port queue priority for packet types | Yes | Yes | |
| Carry 802.1p priority assignment to next downstream device | Yes | No | |
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Carry DSCP policy to downstream devices. The policy includes:
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Yes[b] | Yes[b] | |
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[a] This priority corresponds to the 802.1p priority scheme and is used to determine the packet’s port queue priority. When used in a VLAN-tagged environment, this priority is also assigned as the 802.1p priority carried outbound in packets having an 802.1Q field in the header. [b] Except for non-IPv4 packets or packets processed using QoS IP Precedence, which do not include the DSCP policy option. Also, to use a service policy in this manner, the downstream devices must be configured to interpret and use the DSCP carried in the IP packets. |
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Determine the QoS policy you want to implement. This includes analyzing the types of traffic flowing through your network and identifying one or more traffic types to prioritize. In order of QoS precedence, these are:
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UDP/TCP applications
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Device Priority—destination or source IP address (Note that destination has precedence over source. See QoS marking supported by QoS classifiers.)
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IP ToS Precedence Bits (Leftmost three bits in the ToS field of IP packets)
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IP ToS Differentiated Service bits (Leftmost six bits in the ToS field of IP packets)
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Layer 3 Protocol Priority
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VLAN Priority (requires at least one tagged VLAN on the network)
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Source-Port
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Incoming 802.1p Priority (requires at least one tagged VLAN on the network). In a tagged VLAN environment, the incoming 802.1p priority is used as the default QoS classifier if no QoS classifier with a higher precedence matches
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Select the QoS option you want to use. QoS marking supported by QoS classifiers lists the traffic types (QoS classifiers) and the QoS options you can use for prioritizing or setting a policy on these traffic types:
QoS marking supported by QoS classifiers
Global QoS classifiers Type of QoS marking used to prioritize outbound traffic 802.1p Priority[a] only DSCP policy[b]: DSCP codepoint with 802.1p priority UDP/TCP Supported Supported IP Device Supported Supported IP Precedence Supported[c] Not Supported IP DiffServ Supported Supported L3 Protocol Supported Not Supported VLAN ID Supported Supported Source Port Supported Supported [a] When you configure only the 802.1p priority to mark packets that match a global QoS classifier, the selected traffic is prioritized and sent to the corresponding outbound port queue on the switch. VLAN-tagged ports are necessary to carry the 802.1p priority in a packet header to downstream devices.
[b] When you configure a DSCP policy to mark packets that match a global QoS classifier, the selected traffic is also prioritized according to the associated 802.1p priority and sent to the corresponding outbound port queue on the switch. VLAN-tagged ports carry the 802.1p priority in a packet header to downstream devices. In addition, you can configure downstream devices to read the DSCP value in IP packets and implement the service policy implied by the codepoint.
[c] When using a global QoS IP Precedence classifier, the 802.1p priority is automatically assigned to matching packets based on the IP precedence bit set in the packet header.
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If you want 802.1p priority settings to be included in outbound packets, ensure that tagged VLANs are configured on the appropriate downstream links.
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Determine the actual QoS configuration changes you will need to make on each QoS-capable device in your network in order to implement the desired policy. Also, if you want downstream devices to read and use DSCPs in IP packets from the switch, configure them to do so by enabling ToS Differentiated Service mode and making sure the same DSCP policies are configured.
If ToS Diff-Services mode is enabled, then thirteen rules are immediately used for these codepoints. Adding a new DSCP policy (for example, 001111 with a “5” priority) and then configuring ToS Diff-Services to assign inbound packets with a codepoint of 001010 to the 001111 policy implements all policies configured in the DSCP map and, in this case, uses 13 rules—one for each codepoint invoked in the switch’s current DSCP configuration (the 13 default codepoints). Adding another Diff-Services assignment, such as assigning inbound packets with a codepoint of 000111 to the Expedited Forwarding policy (101110), would use one more rule on all ports.