Fabric Manager Users Guide, Version 6.1, Revision A - QLogic

2–Advanced Fabric Manager Capabilities
Fabric Unicast Routing
Fabric Unicast Routing
Credit Loops
One of the most complex aspects of configuring a large fabric is routing. The SM
must configure a routing for the fabric which provides a careful balance between:
• Performance
• Resiliency to fabric disruptions
• Avoidance of deadlocks and credit loops
• Conservations of resources, such as LIDs and routing table entries
As a result of these sometimes conflicting goals, the SM allows for user
configuration of many aspects of routing so the adminstrator can select the criteria
for routing the fabric.
Since InfiniBand has credit based link layer flow control, credit loops are possible.
Under high stress, a credit loop can become a fabric deadlock which will force
switch timers to discard packets. These deadlocks and discards can cause
significant performance impacts.
The deadlocks are very rare and in practice, they only occur under high bandwidth
applications, however it is better to route the fabric to avoid credit loops altogether.
There are many research papers on the topics of routing. Credit loop avoidance is
a focus of many of the algorithms. Credit loops are avoidable for all the popular
fabric topologies and the SM utilizes algorithms that are designed to avoid credit
loops.
Routing Algorithm
The SM supports the following routing algorithms:
• shortest path
• fattree — optimized balanced routing for fat tree topologies
• Dimension Ordered Routing — Up/Down (dor-updown)
The routing algorithm is selectable using the RoutingAlgorithm parameter.
Shortest Path
This algorithm is the default and works very well for most fabrics. This algorithm
always routes using a least cost path. In most fabrics there are many equal cost
paths, in which case the SM will statically balance the number of paths using each
Inter-Switch Link (ISL).
IB0054608-01 B 2-3