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PowerConnect B-Series FCX Configuration Guide 595
53-1002266-01
QoS for stackable devices
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• DSCP to internal forwarding priority mapping – You can change the mapping between the
DSCP value and the internal forwarding priority value from the default values shown in
Table 94 through Table 97. This mapping is used for CoS marking and determining the internal
priority when the trust level is DSCP. Refer to “Changing the DSCP to internal forwarding priority
mappings” on page 601.
• VLAN priority (802.1p) to hardware forwarding queue - You can change the mapping between
the 802.1p value and hardware forwarding queue from the default value. Refer to “Changing
the VLAN priority 802.1p to hardware forwarding queue mappings” on page 602.
QoS for stackable devices
PowerConnect units in an IronStack support QoS. Units in a stack communicate the stack topology
information and other proprietary control information through the stacking links. For more
information about stacking links and IronStack technology, refer to Chapter 5, “Stackable Devices”.
In addition to control information, the stacking links also carry user network data packets. In an
IronStack topology, the priority of stacking-specific control packets is elevated above that of data
path packets, preventing loss of control packets, and timed retries that affect performance. This
prioritization also prevents stack topology changes that may occur if enough stack topology
information packets are lost.
IronStack technology reserves one QoS profile to provide a higher priority for stack topology and
control traffic.
QoS profile restrictions in an IronStack
In a stacking topology, because CoS level 7 is reserved for stacking, quality profiles for qosp7
cannot be configured. If an attempt is made to configure a profile for qosp7, the system ignores the
configuration.
NOTE
This applies only when the device is operating in stacking mode. It does not apply to standalone
devices.
QoS behavior for trusting Layer 2 (802.1p) in an IronStack
By default, Layer 2 Trust is enabled. Because priority 7 is reserved for stacking control packets, any
ingress data traffic with priority 7 is mapped to internal hardware queue 6. All other priorities are
mapped to their corresponding queues.
QoS behavior for trusting Layer 3 (DSCP) in an IronStack
When the trust dscp mode is enabled, packets arriving with DSCP values 56 to 63 are mapped to
internal hardware queue 6. All other DSCP values are mapped to their corresponding internal
hardware queues.