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74 Fabric OS Administrator’s Guide

53-1002446-01

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.

If even one switch in the fabric delivers out-of-order exchanges, then exchanges are delivered to the

target out of order, regardless of the policy configured on other switches in the fabric.

NOTE

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.

By default, out-of-order frame-based delivery is allowed to minimize the number of frames dropped.

Enabling in-order delivery (IOD) guarantees that frames are either delivered in order or dropped.

You should only force in-order frame delivery across topology changes if the fabric contains

destination devices that cannot tolerate occasional out-of-order frame delivery.

Forcing in-order frame delivery across topology changes

1. Connect to the switch and log in using an account with admin permissions.

2. Enter the iodSet command.

NOTE

The iodSet command can cause a delay in the establishment of a new path when a topology

change occurs; use it with care.

3. Confirm the in-order delivery has been set by entering the iodShow command.

Restoring out-of-order frame delivery across topology changes

1. Connect to the switch and log in using an account with admin permissions.

2. Enter the iodReset command.

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