Qos, Dscp, And Vlans; Dscp Quality Of Service; Vlans And Layer Two Quality Of Service; When Both Dscp And L2Cos Are Used - Brocade Communications Systems Brocade BladeSystem 4/24 User Manual

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:
•
•

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 26
DSCP priority/bits
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Layer three DiffServ code Points (DSCP).
VLAN tagging and Layer two class of service (L2CoS).
Default Mapping of DSCP priorities to L2Cos Priorities
L2CoS priority/bits
7 / 111
1 / 001
Assigned to:
Class F
Medium QoS
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