Quality of Service Components
QoS consists of the following key components:
QoS Component | Description |
---|---|
Classification |
Classification is the process of distinguishing one type of traffic from another based upon access control lists (ACLs), Differentiated Services Code Point (DSCP), Class of Service (CoS), and other factors. |
Marking |
Marking is used on traffic to convey specific information to a downstream device in the network, or to carry information from one interface in a device to another. When traffic is marked, QoS operations on that traffic can be applied. This can be accomplished directly using the set command or through a table map, which takes input values and translates them directly to values on output. Through marking, you can set: a Layer 2 COS value in a frame, Layer 3 DSCP value in a packet, and Layer 3 ECN value in a packet. |
Shaping and Policing |
Shaping is the process of imposing a maximum rate of traffic, while regulating the traffic rate in such a way that downstream devices are not subjected to congestion. Shaping in the most common form is used to limit the traffic sent from a physical or logical interface. Policing is used to impose a maximum rate on a traffic class. If the rate is exceeded, then a specific action is taken as soon as the event occurs. |
Queuing |
Queuing is used to prevent traffic congestion. Traffic is sent to specific queues for servicing and scheduling based upon bandwidth allocation. Traffic is then scheduled or sent out through the port. |
Bandwidth |
Bandwidth allocation determines the available capacity for traffic that is subject to QoS policies. |
Trust |
Trust enables traffic to pass through the device, and the Differentiated Services Code Point (DSCP), precedence, or CoS values coming in from the end points are retained in the absence of any explicit policy configuration. |
Class of Service (CoS) |
CoS refers to the three bits in an 802.1Q header that are used to indicate the priority of the Ethernet frame as it passes through a switched network. The CoS bits in the 802.1Q header are commonly referred to as the 802.1p bits. 802.1Q is discarded prior to frame encapsulation in a VXLAN header, where CoS value is not present in VXLAN tunnel. To maintain QoS when a packet enters the VXLAN tunnel, the type of service (ToS) and CoS values map to each other. |
Differentiated Services Code Point (DSCP) |
The first six bits of the ToS byte in the IP header. DSCP is only present in an IP packet. |
Explicit Congestion Notification (ECN) |
The last two bits of the ToS byte in the IP header. ECN is only present in an IP packet. |