GE T1000, Technical Manual

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GE Reason Switches Chapter 4
– Functions
REASON SWITCHES-TM-EN-3 99

Frame type:
o
Specific Ethertype to CoS queue;
o
Specific LLC parameters to CoS queue;
o
SNAP PID value to CoS queue;
o
IP packets (both IPv4 and IPv6).
 Specific or a range of source IP addresses to CoS queue;
 Specific or a range of source and destination TCP ports to
CoS queue;

Specific or a range of source and destination UDP ports to
CoS queue;
 Specific or a range of DSCP value to CoS queue.
Thus, if Sampled Values and GOOSE messages are in the same VLAN, it is possible to
prioritize one of them over another using its Ethertype, MAC address destination and
so on. Besides, for NTP protocol, it is possible to prioritize it using its UDP port to
redirect this traffic to a specific CoS queue. There are several ways to do traffic
priority services if CoS bits of the incoming data cannot be directly used. Reason
Switches allow up to 256 rules at the QoS Control list, which perform this function.
One major QoS function is the prevention of over flux of messages in a given port,
which can occur at unicast, broadcast or multicast transport mechanisms. These
functions are called Storm policers, and can be performed by Reason switches. Each
port can be configured independently to limit bandwidth in Unicast, Multicast and
Broadcast messages.
Finally, if strict priority has not the performance required at a given LAN, then
Weighted Random Early Detection (WRED) function can be used as a method to
discard frames based on previous assumptions. To choose which frame could be
dropped in this mechanism it is used the DEI bits in an 802.1Q frame. Thus, packets
with DEI 0 are never chosen to be dropped, and packets with DEI 1 are drop-eligible
packets. It is allowed to choose up to 3 queues to have congestion detection, from 0
to 5 priorities CoS queues. CoS 6 and CoS 7priorities queues cannot be used. The
selection of the queues to be used for congestion detection is made at the ingress
port classification, where each port is classified at a drop precedence level (DPL),
between 0 and 3. DPL equal 0 means that these packets cannot be lost and DPL level
1, 2 or 3 mean that packets are eligible to be dropped.
Enabling a queue will permit the switch to perform the WRED function at that queue.
There are, then, four fields that must be configured so the WRED can be executed
correctly: Minimum Threshold, Maximum Drop Probability 1, 2 and 3.
Minimum Threshold field represents the average filling level of the queue which the
switch will start dropping frames randomly. Frames with DEI = 1 in the queue will be
drop-eligible packets. Thus, before there is congestion at the network, the switch will