Brocade Network Advisor SAN User Manual v11.1x (53-1002167-01, May 2011)

642 Document Title
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QOS, DSCP, and VLANs
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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 three DiffServ code Points (DSCP).
• VLAN tagging and Layer two class of service (L2CoS).
DSCP quality of service
Layer three class of service DiffServ Code Points (DSCP) refers to a specific implementation for
establishing QoS policies as defined by RFC2475. 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 may be configured with different DSCP values. Before
configuring DSCP settings, determine if the IP network you are using implements PHB, and consult
with your WAN administrator to determine the appropriate DSCP values.
VLANs and layer two 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 LAN network. A VLAN may reside within a single physical network, or it may 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 down 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 two 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.
When both DSCP and L2CoS are used
If an FCIP tunnel or circuit is VLAN tagged, both DSCP and L2CoS are relevant, unless the VLAN is
end-to-end, with no intermediate hops in the IP network. The following table shows the default
mapping of DSCP priorities to L2Cos priorities. This may be helpful when consulting with the WAN
administrator. These values may be modified per FCIP tunnel.
TABLE 40 Default Mapping of DSCP priorities to L2Cos Priorities
DSCP priority/bits L2CoS priority/bits Assigned to:
46 / 101110 7 / 111 Class F
7 / 000111 1 / 001 Medium QoS