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In-Order Delivery
Reordering SAN Port Channel Frames
When a link change occurs in a SAN port channel, the frames for the same exchange or the same flow can
switch from one path to another faster path (See the following figure).
Figure 40: Link Congestion Delivery
In the figure above, the port of the old path (red dot) is congested. In this scenario, Frame 3 and Frame 4 can
be delivered before Frame 1 and Frame 2.
When the in-order delivery feature is enabled and a port channel link change occurs, the frames crossing the
SAN port channel are delivered as follows:
• Frames using the old path are delivered before new frames are accepted.
• The new frames are delivered through the new path after the network latency drop period has elapsed
and all old frames are flushed.
Frames that cannot be delivered in order through the old path within the network latency drop period are
dropped.
Related Topics
Configuring the Drop Latency Time, on page 182
About Enabling In-Order Delivery
You can enable IOD for a specific VSAN or for the entire switch. By default, IOD is disabled on Cisco SAN
switches.
We recommend that you enable this feature only when devices that cannot handle any out-of-order frames
are present in the switch. Load-balancing algorithms within the switch ensure that frames are delivered in
order during normal fabric operation. The load-balancing algorithms based on source FC ID, destination FC
ID, and exchange ID are enforced in the hardware without any performance degradation. However, if the
fabric encounters a failure and the in-order delivery feature is enabled, the recovery will be delayed because
of an intentional pausing of fabric forwarding to purge the fabric of resident frames that could potentially be
forwarded out-of-order.
Enabling In-Order Delivery
You can enable in-order delivery for the switch.
Cisco Nexus 5500 Series NX-OS SAN Switching Configuration Guide, Release 7.x
180
Configuring Fibre Channel Routing Services and Protocols
OL-30895-01
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