Iscsi Routing Modes - Cisco AP775A - Nexus Converged Network Switch 5010 Configuration Manual

Chapter 50 Configuring iSCSI
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m
You see the Ethernet Interfaces and iSCSI dialog box (see
Click the iSCSI TCP tab in either Fabric Manager or Device Manager.
Step 3
You see the iSCSI TCP configuration table.
Set the QoS field from 1 to 6.
Step 4
Click the Apply Changes icon in Fabric Manager or click Apply in Device Manager to save these
Step 5
changes.

iSCSI Routing Modes

Cisco MDS 9000 Family switches support multiple iSCSI routing modes. Each mode negotiates different
operational parameters, has different advantages and disadvantages, and is suitable for different usages.
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Figure 50-29
OL-17256-03, Cisco MDS NX-OS Release 4.x
Pass-thru mode
In pass-thru mode, the port on the IPS module or MPS 14/2 module converts and forwards read data
frames from the Fibre Channel target to the iSCSI host frame-by-frame without buffering. This
means that one data-in frame received is immediately sent out as one iSCSI data-in PDU.
In the opposite direction, the port on the IPS module or MPS 14/2 module limits the maximum size
of iSCSI write data-out PDU that the iSCSI host can send to the maximum data size that the Fibre
Channel target specifies that it can receive. The result is one iSCSI data-out PDU received sent out
as one Fibre Channel data frame to the Fibre Channel target.
The absence of buffering in both directions leads to an advantage of lower forwarding latency.
However, a small maximum data segment length usually results in lower data transfer performance
from the host because of a higher processing overhead by the host system. Another benefit of this
mode is iSCSI data digest can be enabled. This helps protect the integrity of iSCSI data carried in
the PDU over what TCP checksum offers.
Store-and-forward mode (default)
In store-and-forward mode, the port on the IPS module or MPS 14/2 module assembles all the Fibre
Channel data frames of an exchange to build one large iSCSI data-in PDU before forwarding it to
the iSCSI client.
In the opposite direction, the port on the IPS module or MPS 14/2 module does not impose a small
data segment size on the host so the iSCSI host can send an iSCSI data-out PDU of any size (up to
256 KB). The port then waits until the whole iSCSI data-out PDU is received before it converts, or
splits, the PDU, and forwards Fibre Channel frames to the Fibre Channel target.
The advantage of this mode is higher data transfer performance from the host. The disadvantages
are higher transfer latency and that the iSCSI data digest (CRC) cannot be used.
The store-and-forward mode is the default forwarding mode.
Cut-through mode
Cut-through mode improves the read operation performance over store-and-forward mode. The port
on the IPS module or MPS 14/2 module achieves this by forwarding each Fibre Channel data-in
frame to the iSCSI host as it is received without waiting for the whole exchange complete. There is
no difference for write data-out operations from store-and-forward mode.
compares the messages exchanged by the iSCSI routing modes.
Figure 50-21).
Cisco MDS 9000 Family Fabric Manager Configuration Guide
Configuring iSCSI
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