Hardware Reference Manual
CP945FP-12 CompactPCI, RM945FP-12 VME, & RM948 With Embedded Layer 2
Switch, Front Input/Output
DDC No. Rx-URMH 122 Rev A
Issued 5 December 2003
8 of 18
2.1 Operational Configuration Options
The switch fabric has a number of performance operations usually found only in managed switch products.
These options include:
•
Port based VLAN
•
QoS
•
Port speed/duplex forced configuration
All of these parameters are set via the SROM. This can either be programmed prior to manufacturing or
configured in the field from the front panel parallel port.
2.1.1 Port Based VLAN
The switch fabric provides the designer with the ability to define a single port-based VLAN for each of the 12
ports. This VLAN is individually defined for each port, assigning a VLAN ID (value between 0–11) for each port.
When packets arrive at an input of the switch, the search engine will determine the VLAN ID for that port, and will
then determine which of the other ports are also members of that VLAN by matching their assigned VLAN ID
values. The packet will then be transmitted to each port with the same VLAN ID as the source port.
2.1.2 QoS
QoS provides a new level of capability to unmanaged switch applications with two transmit queues per output
port.
The fabric manages the output transmission queues for all ports of the fabric. Once the destination address
search is complete, the packet is inserted into the appropriate output queue. Packet entry into high or low priority
queue is controlled by either the VLAN tag information or the TOS field in the IP header. Each of these priority
fields can used to select the transmission queue priority as well as a packet drop probability. The mapping of the
tag and TOS fields to either the high or low priority queue is configured on power-up from the SROM.
The fabric utilizes Weighted Round Robin (WRR), Random Early Drop with In/Out (RED/RIO) bit, and a
timestamp method of scheduling packets for transmission. WRR uses an efficient method to ensure that each of
the transmission queues gets at least a minimum service level. With two output transmission queues, the fabric
will transmit “X” packets from the high priority queue before transmitting “Y” packets from the low priority queue.
The “X” and “Y” weights are user definable. The high priority weight can be set to a value between 0-16. The low
priority weight is fixed at the value 1. If the high priority weight is set to the value 4, then the fabric will transmit
four high priority packets before transmitting each low priority packet.
The timestamp method of scheduling packets for transmission allows the fabric to offer latency guarantees to high
priority packets. This is ideal for voice and video packets that have strict latency requirements. Packets are given
a timestamp when they arrive at the input port. Once they are scheduled into a transmission queue, the fabric will
keep track of the packets delay time through the chip. Under heavy congestion the timestamp may be utilized to
change the order of packet transmission to ensure timely delivery of packets and orderly service of congested
queues.
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