SM ALFOplus2 - Queues and Congestion Avoidance Methods; Scheduling

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• S-TAG + C-TAG: (Bandwidth Profiles per S_VID+CoS): a different profile is defined for different
couples S_VID+priority queue (up to 64 different cases can be managed).
In general different criteria can be defined for each port/VLAN/priority.
Up to 64 Ingress Filtering Policy resources can be defined and each bandwidth profile defined on the basis
either of LAN port, VLAN or VLAN+priority consumes 1 of such resources.
In order to define the bandwidth profile, the following parameters must be configured:
• CIR (Committed Information Rate): it is the admitted ingress rate (“green” colored), with values
between 0 Kbit/s and 1 Gbit/s.
• CBS (Committed Burst Rate): it is the maximum size of the token bucket of the green packets, with
values between 0 byte and 256 Kbyte.
• EIR (Excess Information Rate): it is maximum ingress rate eventually admitted (“yellow” colored),
with values between 0Kbit/s and 1Gbit/s.
• EBS (Excess Burst Rate): it is the maximum size of the token bucket of the yellow packets, with
values between 0 byte and 256 Kbyte.
5.8.6 Queues and Congestion avoidance methods
When the queues are full there is a congestion situation, this means that the resources are not enough to
serve all the packets and some of them must be dropped.
The choice of the dropping policy has different effects on the network. In case of congestion, the TCP/IP
protocol reduces the transmitting windows and therefore the amount of traffic transmitted. The TCP/IP pro-
tocol increases the transmitting window very slowly to allow the network to solve the congestion issues.
Congestion Avoidance is a protocol that permits to discard some frames before congestion occurs.
Supported mechanisms are:
• Random Early Detection (RED)
• Weighted Random Early Detection (WRED)
•Tail Drop.
5.8.7 Scheduling
Once the priority is assigned, the traffic in the queues is then emptied by means of either Strict Priority or
Weight Round Robin algorithms:
• Strict Priority - the highest priority takes always precedence.
• WRR - the available bandwidth is shared among the different priorities with configurable weights
It is also possible to configure at the same time some queues as Strict Priority and the remaining as WRR.
ALFOPLUS2 ethernet switch supports 8 queues per port with configurable depth.