Alcatel Carrier Internetworking Solutions 8800 User Manual

OS6624/6648 Architecture

OmniSwitch Troubleshooting Guide

page -53

• Several types of priority rules are supported. Each rule is associated with a specific device priority. If a 

packet satisfies a certain priority rule, it is assigned the device priority corresponding to that rule.

• Global Priority Rules enable assignment of specific device priority to all packets from a specific source 

address (global source priority) or all packets meant for a specific destination address (global destina-
tion priority). This is enabled through the Rules Entry data structure.

• Flow-based Priority Rules enable assignment of priorities by setting up priority flows. A flow refers to 

packet transmission between a specific source and a specific destination address. When a packet is 
detected to belong to a priority flow, it may be assigned a device priority as a set up for the flow. This 
is enabled through the Rules Entry data structure.

• Protocol-based Priority is enabled by Flow-Based and Global Priority. It allows the user to assign 

different device priority for packets that satisfy the same flow rules or global priority rules, but corre-
spond to different protocols.

The QoS-specified device priority may override the device priority already assigned to a packet through 
the VLAN tag, or through the global or flow-based priority. The decision to override previously assigned 
device priority in favor of QoS-specified device priority is based on the value of bit 30 of SIC Control 0 
Register. The user configures this bit. 

• The IXE2424 provides Bandwidth Management at two levels. First, the device manages bandwidth 

between different output queues (that store pointers to the packet entries for transmission) at each port 
through a priority queuing scheme. Credit based and strict priority are supported. The choice of one of 
these two algorithms is configured on a per port basis using the Weighted Fair Queuing Port Address 
Control Registers. 

• The second level of Bandwidth Management is at the per-output queue level, through QoS rules for 

packet flows, setup by the user. This provides bandwidth management on per queue and per flow basis 
and is termed QoS bandwidth management. The corresponding flows are termed QoS flows. A QoS 
flow essentially specifies a traffic-policing rule and allows users to limit bandwidth assigned for the 
specific flow. QoS flows may be specified in terms of flow between two specific addresses, A and B; 
flow from a specific address to any destination address; and flow to a specific destination address from 
any source address.

• A QoS flow specifies a data limit value, time interval over which the data limit is to be enforced, and a 

priority level (which determines the transmit queue number) for the flow. The IXE2424 device keeps 
track of the amount of data that has been transmitted within the user-specified time intervals for each 
QoS flow. If a packet causes a specific QoS flow to exceed its data allocation, it is dropped. Multiple 
QoS flows may be mapped to the same transmit queue and still be guaranteed the required bandwidth 
for that flow - the bandwidth management feature takes care of guaranteeing the bandwidth for the 
queue (by not allowing other queues to take away unauthorized bandwidth) and the QoS feature guar-
antees bandwidth for all flows mapped to the same queue (by not allowing any of the flows to exceed 
their allocated data rate). The device level Bandwidth Management feature is responsible for guaran-
teeing the bandwidth for an output queue (by not allowing other queues to take away unauthorized 
bandwidth) and the QoS feature guarantees the bandwidth for all flows mapped to the same queue (by 
not allowing other flows to exceed their allocated data rate). By combining these two types of band-
width management, users can efficiently manage bandwidth for different types of traffic with the 
IXE2424 device.