Dscp quality of service, Vlans and layer two quality of service, When both dscp and l2cos are used – Brocade Network Advisor SAN User Manual v12.3.0 User Manual

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Brocade Network Advisor SAN User Manual

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QOS, DSCP, and VLANs

22

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 75

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

11 / 001011

3 / 011

Medium QoS

15 / 001111

3 / 011

Medium QoS

19/ 010011

3 / 011

Medium QoS

23 / 010111

3 / 011

Medium QoS

27 / 011011

0 / 000

Class 3 Multicast

31 / 011111

0 / 000

Broadcast/Multicast

35 / 100011

0 / 000

Low Qos

39 / 100111

0 / 000

Low Qos

43 / 101011

4 / 100

High QoS

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