HP StorageWorks Fabric OS 6.1.1 administrator guide (5697-0235, December 2009)

Fabric OS 6.1.1 administrator guide 239
Frame order delivery
The order of delivery of frames is maintained within a switch and determined by the routing policy in effect.
The frame delivery behaviors for each routing policy are:
• Port-based routing
All frames received on an incoming port destined for a destination domain are guaranteed to exit the
switch in the same order in which they were received.
• Exchange-based routing
All frames received on an incoming port for a given exchange are guaranteed to exit the switch in the
same order in which they were received. Because different paths are chosen for different exchanges,
this policy does not maintain the order of frames across exchanges.
NOTE: Some devices do not tolerate out-of-order exchanges; in such cases, use the port-based routing
policy.
If even one switch in the fabric delivers out-of-order exchanges, exchanges are delivered to the target
out-of-order, regardless of the policy configured on other switches in the fabric.
Some devices do not tolerate out-of-order exchanges; in such cases, use the port-based routing policy.
In a stable fabric, frames are always delivered in order, even when the traffic between switches is shared
among multiple paths. However, when topology changes occur in the fabric (for example, if a link goes
down), traffic is rerouted around the failure, and some frames could be delivered out of order. Most
destination devices tolerate out-of-order delivery, but some do not.
Use of out-of-order frame-based delivery can minimize the number of frames dropped, while enabling
in-order delivery (IOD) guarantees that frames are either delivered in order or dropped. All HP B-series
switches enable IOD by default to ensure frames are delivered in order, as not all HP destination devices
can tolerate out-of-order frames.
Disabling in-order frame delivery across topology changes
1. Connect to the switch and log in as admin.
2. Issue the iodReset command at the command line.
NOTE: The iodReset command can cause a delay in the establishment of a new path when a topology
change occurs; use it with care.
3. To confirm the in-order delivery has been disabled, issue the iodShow command.
Restoring in-order frame delivery across topology changes
1. Connect to the switch and log in as admin.
2. Issue the Set command at the command line.
Dynamic load sharing
The exchange-based routing policy depends on the Fabric OS Dynamic Load Sharing feature (DLS) for
dynamic routing path selection. When using the exchange-based routing policy, DLS is by default enabled
and cannot be disabled. In other words, you cannot enable or disable DLS when the exchange-based
routing policy is in effect.
When the port-based policy is in force, you can enable DLS to optimize routing. When DLS is enabled, it
shares traffic among multiple equivalent paths between switches. DLS recomputes load sharing when any
of the following occurs:
• A switch boots up
• An E_Port goes offline and online
• An EX_Port goes offline