User guide

Switching Layer 2 switch functional description
iMG/RG Software Reference Manual (Switching)
2-4
To detect a loop on ethernet ports, the Gateway periodically sends a “special” ping message. If the gateway
receives the same ping message back, it means that a loop is present. In this case the Gateway disables all the
traffic to/from the port (except the “special” ping) until the loop has been removed.
2.2.2.6 Layer 3 Routing Rate Limiting
The integrated layer 2 switch can limit traffic that goes to the Gateway network processor where routing tasks
need to be performed.
Limitation on the maximum routing rate is necessary to preserve system resources for high priority tasks like
VoIP and IGM P.
If the number of frames per seconds that need to be routed to the network processor are higher than the
selected maximum rate, the layer 2 switch discards packets addressed to the network processor in order to
force the average traffic rate to be below the target rate.
2.2.2.7 Quality of Service Classification
QoS switching policy is performed by the Queue Controller. The priority of a frame is determined in priority
order by:
• The IEEE 802.3ac Tag containing IEEE 802.1p priority information: this IEEE 802.1p priority information is
used in determining frame priority when IEEE 802.3ac tagging is enabled on the port.
• The IPv4 Type of Service (TOS)/DiffServ field when enabled on the port. IPv4 priority classification can be
configured on a port basis to have a higher priority then IEEE Tag.
The user can enable these classification individually or in combination.
All untagged frames entering a port have their priority set to the port's default priority. This priority is then
used to manage the traffic from that port.
There are two different models in place:
1. A two Queue scheme- where by the user specifies which Priority settings go into the high priority queue and
which go into the low queue.
2. A four Queue scheme where the user actually maps the different priority values to one of the four queues.
Highest priority queues are emptied first before the lower priority queues…and as such, it is possible for the
low priority traffic to get starved out.
The integrated layer 2 switch supports two Class of Service (CoS) mechanisms: IEEE 802.1p tagging (Layer 2)
and Differentiated Services (DS) as an advanced architecture of ToS (Layer 3).
2.2.2.7.1 802.1p traffic priority
The IEEE 802.1p signalling technique is an IEEE endorsed specification for prioritizing network traffic at the
data-link/MAC sub-layer (OSI Reference Model Layer 2).