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QoS & Layer-2/Layer-3/Layer-4 Flow Overview

312 Enterasys Xpedition User Reference Manual

Within the XP, QoS policies are used to classify Layer-2, Layer-3, and Layer-4 traffic into

the following priority queues (in order from highest priority to lowest):

• Control (for router control traffic; the remaining classes are for normal data flows)

• High

• Medium

• Low

Separate buffer space is allocated to each of these four priority queues. By default,

buffered traffic in higher priority queues is forwarded ahead of pending traffic in lower

priority queues (this is the strict priority queuing policy). During heavy loads, low-priority

traffic can be dropped to preserve the throughput of the higher-priority traffic. This

ensures that critical traffic will reach its destination even if the exit ports for the traffic are

experiencing greater-than-maximum utilization. To prevent low-priority traffic from

waiting indefinitely as higher-priority traffic is sent, you can apply the weighted fair

queuing (WFQ) queuing policy to set a minimum bandwidth for each class. You can also

apply weighted random early detection (WRED) to keep congestion of TCP traffic under

control.

Layer-2 and Layer-3 & Layer-4 Flow Specification

In the XP, traffic classification is accomplished by mapping Layer-2, -3, or -4 traffic to one

of the four priorities. Each traffic classification is treated as an individual traffic flow in the

XP.

For Layer-2 traffic, you can define a flow based on the following:

• MAC packet header fields, including source MAC address, destination MAC address

and VLAN IDs. A list of incoming ports can also be specified.

For Layer-3 (IP and IPX) traffic, you can define “flows”, blueprints or templates of IP and

IPX packet headers.

• The IP fields are source IP address, destination IP address, UDP/TCP source port,

UDP/TCP destination port, TOS (Type of Service), transport protocol (TCP or UDP),

and a list of incoming interfaces.

• The IPX fields are source network, source node, destination network, destination node,

source port, destination port, and a list of incoming interfaces.

For Layer-4 traffic, you can define a flow based on source/destination TCP/UDP port

number in addition to Layer-3 source/destination IP address.

The flows specify the contents of these fields. If you do not enter a value for a field, a

wildcard value (all values acceptable) is assumed for the field.

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Cabletron Systems SSIM-R8-02 Spécifications Page 332