Reference Guide

24 Fabric OS FCIP Administrator’s Guide

53-1002474-01

QoS, DSCP, and VLANs

Quality of Service (QoS) refers to policies for handling differences in data traffic. These policies are

based on data characteristics and delivery requirements. For example, ordinary data traffic is

tolerant of delays and dropped packets, but voice and video data are not. QoS policies provide a

framework for accommodating these differences in data as it passes through a network.

QoS for Fibre Channel traffic is provided through internal QoS priorities. Those priorities can be

mapped to TCP/IP network priorities. There are two options for TCP/IP network-based QoS:

• Layer 3 Differentiated Services Code Point (DSCP)

• VLAN tagging and Layer 2 Class of Service (L2CoS)

DSCP Quality of Service

Layer 3 Class of Service Differentiated Services Code Point (DSCP) refers to a specific

implementation for establishing QoS policies as defined by RFC 2475. DSCP uses six bits of the

Type of Service (TOS) field in the IP header to establish up to 64 different values to associate with

data traffic priority.

DSCP settings are useful only if IP routers are configured to enforce QoS policies uniformly within

the network. IP routers use the DSCP value as an index into a Per Hop Behavior (PHB) table. Control

connections and data connections can be configured with different DSCP values. Before

configuring DSCP settings, determine if the IP network you are using implements PHB, and consult

with the WAN administrator to determine the appropriate DSCP values.

VLANs and Layer 2 Quality of Service

Devices in physical LANs are constrained by LAN boundaries. They are usually in close proximity to

each other, and share the same broadcast and multicast domains. Physical LANs often contain

devices and applications that have no logical relationship. Also, when logically related devices and

applications reside in separate LAN domains, they must be routed from one domain to the other.

A VLAN is a virtual local area network. A VLAN can reside within a single physical network, or it can

span several physical networks. Related devices and applications that are separated by physical

LAN boundaries can reside in the same VLAN. Also, a large physical network can be broken into

smaller VLANs. VLAN traffic is routed using 802.1Q-compliant tags within an Ethernet frame. The

tag includes a unique VLAN ID, and Class of Service (CoS) priority bits. The CoS priority scheme

(also called Layer 2 Class of Service or L2CoS) uses three Class of Service (CoS or 802.1P) priority

bits, allowing eight priorities. Consult with your WAN administrator to determine usage.