Specifications
DATA CENTER BEST PRACTICES
SAN Design and Best Practices 43 of 84
In a shared link situation, if you think of the bandwidth as separated into three areas, black (0 Ò x bps), gray
(x Ò y bps), and white (y Ò maximum bps), ARL can help manage the bandwidth usage. Black is the oor value
for ARL. This is the amount of bandwidth that is reserved exclusively for FCIP. White is the ceiling value, and it
is reserved exclusively for other shared trafc. Gray is the area in between, which FCIP may use if other shared
trafc is not using it. This other shared trafc can also be another FCIP application such as tape. Black would be
the RDR trafc; white would be tape trafc, and they would adaptively share the gray area. There are many ways
in which you can use ARL. These are just a few popular examples.
PerPriority-TCP-QoS
PP-TCP-QoS is a Brocade innovation (patent pending).
Differentiated Services Code Point (DSCP) is an IP based (L3) Quality of Service (QoS) marking; therefore,
since IP is end-to-end protocol, DSCP is an end-to-end QoS marking. DSCP has 64 values; however, the range
of values from 0 through 63 do not denote the lowest priority through the highest priority. The valuing system
works differently. First, all odd numbers are available for private use and can be used in any way that enterprise
deems valuable. These odd numbers are for private use the same way that RFC 1918 IP addresses are; for
example, 192.168.0.1 and 10.1.2.3 are private IP addresses that can be used in any way an enterprise wishes.
For non-private DSCP values, DSCP value 46 is referred to as Expedited Forwarding and is the highest priority.
Zero is the default, and it is the lowest priority. There are four groups of High/Medium/Low (H/M/L) values
referred to as Assured Forwarding. Another group of numbers has backwards compatibility with legacy ToS
(Type of Service). The selection of DSCP to be used in the IP network is the responsibility of the IP network
administrators. Without their buy-in and conguration of the Per-Hop Behavior (PHB) associated with QoS, no
QoS can actually happen. The default behavior of Ethernet switches is to replace ingress QoS values with the
default value (0), unless the data coming in on that interface is explicitly deemed to be QoS “trusted.” This
prevents end users from setting their own QoS values unannounced to the IP networking administrators.
802.1P is a data link-based (L2) QoS marking; therefore, the scope extends only from the interface of one
device to the interface of the directly attached device. Devices that enforce 802.1P provide QoS across that
data link. 802.1P has a header that resides in the 802.1Q VLAN tagging header; therefore, VLAN tagging is
required to get 802.1P QoS marking. Brocade FOS refers to 802.1P as L2CoS. There are only eight values for
802.1P—from 0 to 7. Zero is the lowest priority and the default. Seven is the highest priority.
The Brocade 7800/FX8-24 supports three levels of priority (H/M/L). The default amount of BW that the
scheduler apportions during times of contention is 50/30/20 percent. QoS portioning of BW occurs only during
times of contention; otherwise, the BW is shared equally across all priorities. It is possible to change the default
portions to any values you wish, as long as High>Middle>Low, and the aggregate of all the priorities equals
100 percent.
There are four TCP sessions per FCIP circuit: H, M, L, and F-Class. F-Class uses a strict queuing, which means
that if there is any F-Class trafc to send, it all gets sent rst. There is very little F-Class trafc, and it does not
interfere with data trafc. Each TCP session is autonomous and does not rely on other TCP sessions or settings.
Each TCP session can be congured with its own DSCP, VLAN tagging, and 802.1P values. This permits that
TCP session (priority) to be treated independently in the IP network from site-to-site, based on the SLA for that
QoS priority.
Brocade has QoS in Brocade FC/FICON fabrics and across FC ISLs via Virtual Channels (VCs). There are
different VCs for H/M/L/F-Class, each with its own set of Buffer-to-Buffer Credits and ow control. There are
ve VCs for high levels, four VCs for medium levels, and 2 VCs for low levels. Devices are assigned to QoS VCs
by enabling QoS on the fabric and then putting the letters QOSH_ or QOSL_ as a prex to the zone name. The
default is QOSM_, so there is no need to explicitly designate medium zones. Once devices are assigned to
these VCs, they use these VCs throughout the fabric. If data ingresses to a Brocade 7800/FX8-24 via an ISL on
a particular VC, the data is automatically assigned to the associated TCP sessions for that priority. Devices that
are directly connected to the Brocade 7800/FX8-24 are also assigned to the associated TCP session priority
based on the zone name prex.