Quality of Service Overview
17-2 Configuring Quality of Service
secondly, you must identify these flows in a way that QoS can recognize. In this sense, QoS is the
third step in a three step process. The three-steps Enterasys recommends for configuring QoS are:
• Understand your network flows using NetFlow
• Associate the flows on your network with a well defined role using Enterasys policy
• Configure the appropriate link behavior for that role by associating the role with a QoS
configuration
Quality of Service Operation
QoS is all about managing the bandwidth in a manner that aligns the delivery requirements of a
given flow with the available port resources. In a QoS context, a flow is a stream of packets that are
classified with the same class of service as the packets transit the interface. QoS manages
bandwidth for each flow by:
• Assigning different priority levels to different packet flows.
• Marking or re-marking the packet priority at port ingress with a Type of Service (ToS).
• Sorting flows by transmit queue. Higher priority queues get preferential access to bandwidth
during packet forwarding.
• Limiting the amount of bandwidth available to a given flow by dropping (rate limiting)
packets in excess of configured limits.
These QoS abilities collectively make up a Class of Service (CoS). The remainder of this section
will describe CoS and its components.
Class of Service (CoS)
You implement QoS features in a Class of Service (CoS). The hardware resource components that
can be configured as part of a CoS are:
• Inbound Rate Limiters (IRL) - allow you to configure a threshold above which a port will not
process traffic.
• Flood Control - configures a threshold above which a port will not receive unknown-unicast,
multicast, or broadcast packets.
The CoS configuration of each port hardware resource is optional. IRL and flood control each have
a single configurable rate limiting port hardware resource option.
CoS configuration is applied to the ingressing packet based upon the packet’s 802.1 priority, port,
and policy settings.
How the firmware treats a packet as it transits the link depends upon the priority and forwarding
treatments configured in the CoS assigned to the packet. Up to 256 unique CoS entries can be
configured. CoS entries 0–7 are configured by default with an 802.1p priority assigned and default
forwarding treatment. CoS entries 0–7 cannot be removed. CoS entries 0–7 are reserved for
mapping an 802.1p priority to a CoS index. CoS entries 8-255 can be configured and used by policy
for the following services:
• 802.1p priority
• IP Type of Service (ToS) marking
•In-bound (IRL) rate limiter
• Flood control
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