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Figure 1.
MSTP Settings screen for the Dell PowerConnect 5316M switch
Figure 1 shows the MSTP Settings screen in the Dell OpenManage
Switch Administrator Web browser–based graphical user interface (GUI)
for the PowerConnect 5316M. Administrators can use this screen to con-
figure the Multiple Spanning Tree (MST) region name, revision number,
and VLAN-to-instance mapping. Adjacent switches must use the same
settings for them to become part of the same MST region. Administrators
can change the bridge priority setting to influence the root bridge selection
for a particular spanning tree instance.
Administrators can use the following commands to manipulate the
cost and priority settings, which play a major role in determining the active
and blocked interfaces in the topology:
console(config-if)# spanning-tree mst
instance_id cost cost
console(config-if)# spanning-tree mst
instance_id port-priority priority
They can execute these commands from the Ethernet or link aggrega-
tion group (LAG) interface configuration mode, and can do so separately
for each MSTP instance. A LAG interface represents the link aggregation
of multiple physical interfaces between two Ethernet switches and appears
to the spanning tree algorithm as a single logical interface.
Classification
Incoming
Inspects incoming packet priority
packet
value and assigns default priority
if appropriate
Figure 2.
QoS flow diagram for the Dell PowerConnect 5316M switch
reprinted from Dell Power Solutions, May 2007. Copyright © 2007 dell inc. all rights reserved.
Using quality of service for traffic management
In practical situations, traffic from blade servers to a chassis switch may
consist of more than the uplink (traffic from the chassis switch to the
external network): the switch accommodates surplus traffic in queues
that act as buffers, which results in network latency and can cause prob-
lems for time-sensitive data such as voice over IP (VoIP) traffic. Quality
of service refers to traffic mechanisms that prioritize data flow for certain
data sources over others, in contrast to a "best-effort" mechanism that
allocates network resources equally to competing applications. QoS in a
Layer 2 switch, such as the PowerConnect 5316M, can thus be referred to
as a priority queuing scheme.
Figure 2 depicts the three phases a packet passes through when it
enters the switch: classification, queuing, and scheduling. Classification
is the process of distinguishing different types of traffic by examining the
packet fields. The PowerConnect 5316M can detect a packet's priority
through two schemes: class of service (CoS) and differentiated services
™
code point (DSCP). CoS is part of the IEEE 802.1p standard and is con-
tained in a three-bit field in the VLAN header; its values range from 0
to 7, with 7 having the highest priority. DSCP is an eight-bit field in the
Layer 3 IP header; its values range from 0 to 63, with 63 having the highest
priority. Default CoS values can be assigned to the incoming frames on
each port. Administrators can set the trust mode on the PowerConnect
5316M, which carries forward the embedded priority in the incoming
frame, to use either CoS or DSCP with the following command (where
scheme is either dscp or cos):
console(config)# qos trust scheme
Queuing is the process of assigning incoming packets to the appropri-
ate egress queue. The switch bases the outgoing port on the forwarding
table entry for the destination MAC address. Each port has four queues,
and the packet is assigned to one of these based on the priority tag. The
following commands map CoS and DSCP values to specific queues:
console(config)# wrr-queue cos-map queue_id
cos_values_list
console(config)# qos map dscp-queue
dscp_values_list to queue_id
Queuing
Assigns packet to egress
queue of appropriate port
queue based on scheduling scheme
www.dell.com/powersolutions
Scheduling
Sends packet out of egress
www.dell.com/powersolutions
Outgoing
packet
2
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