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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

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

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Scheduling

Sends packet out of egress

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Outgoing

packet