CANOGA PERKINS 9175 Configuration Guide - Manual (Page 207)

CanogaOS Configuration Guide
31-1
31 Configuring QoS
31.1 Overview
Quality of Service (QoS) can be used to give certain traffic priority over other traffic.
Without QoS, all traffic in a network has the same priority and chance of being delivered
on time. If congestion occurs, all traffic has the same chance of being dropped.
With QoS, specific network traffic can be prioritized to receive preferential treatment. In
turn, a network performs more predictably, and utilizes bandwidth more effectively.
QoS Functionality
Classification information can be carried in the Layer-3 IP packet header or the Layer-2
frame. IP packet headers carry the information using 6 bits or 3 bits from the deprecated
IP type of service (TOS) field. Layer-2 802.1Q frames carry the information using a
2-byte Tag Control Information field.
All switches and routers accessing the Internet depend on class information to give the
same forwarding treatment to packets with the same class information, and give different
treatment to packets with different class information. A packet can be assigned class
information, as follows:
• End hosts or switches along a path, based on a configured policy
• Detailed packet examination, expected to occur nearer to the network edge, to
prevent overloading core switches and routers
• A combination of the above two techniques
Class information can be used by switches and routers along a path to limit the amount
of allotted resources per traffic class.
Per-hop behavior is an individual device’s behavior when handling traffic in the DiffServ
architecture. An end-to-end QoS solution can be created if all devices along a path have
consistent per-hop behavior.
31.2 Terminology
Following is a brief description of terms and concepts used to describe QoS.
ACL
Access control lists (ACLs) classify traffic with the same characteristics. IP traffic is
classified using IP ACLs, and non-IP traffic is classified using MAC ACLs.
The ACL can have multiple access control entries (ACEs), which are commands that
match fields against the contents of the packet.
CoS Value
Class of Service (CoS) is a 3-bit value used to classify the priority of Layer-2 frames
upon entry into a network.
QoS classifies frames by assigning priority-indexed CoS values to them, and gives
preference to higher-priority traffic.
Layer-2 802.1Q frame headers have a 2-byte Tag Control Information field that carries
the CoS value in the 3 most significant bits, called the User Priority bits. On interfaces
configured as Layer-2 802.1Q trunks, all traffic is in 802.1Q frames, except for traffic in
the native VLAN.