Quality of Service (QoS): Managing Bandwidth More Effectively

Preparation for Configuring QoS

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.

Table 5-5. 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:

Yes1

Yes1

Assigning a ToS Codepoint

 

 

Assigning an 802.1p Priority 2 to the Codepoint

 

 

1Except for non-IPv4 packets or packets processed using either the Layer 3 Protocol or QoS IP-Precedence methods, 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.

2This 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.

Steps for Configuring QoS on the Switch

1.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:

a.IP Type-of-Service Precedence Bits (Leftmost three bits in the ToS field of IP packets)

b.IP Type-of-Service Differentiated Service bits (Leftmost six bits in the ToS field of IP packets)

c.Source-Port

d.Incoming 802.1p Priority (requires at least one tagged VLAN on the network)

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