Preparation for configuring QoS
Preserving 802.1p priority
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 | |
Carry DSCP policy to downstream devices. The policy includes:
|
Yes[b] | Yes[b] | |
[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. | |||
Steps for configuring QoS on the switch
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:
IP Type-of-Service Precedence Bits (Leftmost three bits in the ToS field of IP packets)
IP Type-of-Service Differentiated Service bits (Leftmost six bits in the ToS field of IP packets)
VLAN Priority (requires at least one tagged VLAN on the network)
Source-Port
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
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.
If you want 802.1p priority settings to be included in outbound packets, ensure that tagged VLANs are configured on the appropriate downstream links.
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 that the same DSCP policies are configured.