This physical network connects Ethernet iSCSI adapters in the hosting i5/OS™ with Ethernet iSCSI adapters in the hosted system. It is typically a simple, switched, Gigabit Ethernet network. Two kinds of traffic flow over this connection: storage (SCSI) and virtual Ethernet (LAN).

On one side of the network is an iSCSI adapter or adapters controlled by i5/OS. Each iSCSI adapter has two IP addresses: one for SCSI and one for LAN. You configure the IP addresses and other attributes of an adapter in an i5/OS device description object known as the network server host adapter. For more information, see Network server host adapters. Each iSCSI adapter controlled by i5/OS needs its own object. Every iSCSI adapter contains a TCP/IP stack implemented in hardware that is independent of the normal i5/OS TCP/IP stack. When you vary on a network server host adapter, an iSCSI adapter controlled by i5/OS uses the configured values. If you want different values to take effect, change the configuration and vary on the server host adapter again. The i5/OS TCP/IP stack is unaware of the IP addresses configured for the iSCSI adapters.

On the other side of the network is an iSCSI adapter or adapters for the hosted system. You configure the IP addresses and other attributes of these adapters in an i5/OS object known as the remote system configuration. For more information, see Remote system configuration. This configuration differs from the i5/OS network server adapter object in several ways:

Notes:

In i5/OS configuration objects, network interface information is labeled as local or remote. These terms are relative to i5/OS. Local interface information is for the i5/OS side. Remote interface information is for the Windows hosted system side.
The network server host adapter and the remote system configuration define IP address information for opposite sides of the iSCSI network. When connected by a simple, switched network, the following rules apply:
- The SCSI internet addresses in these two objects that are connected by a switch must be in the same subnet. For example, with IP addresses of the form a.b.x.y and 255.255.255.0 subnet masks, a.b.x must be the same value for both objects.
- The LAN internet addresses in these two objects that are connected by a switch must be in the same subnet.
- In the network server host adapter, the gateway elements can be any unassigned IP address in any subnet if you don't have a gateway in your network.
- In the remote system configuration, the gateway elements should be blank if you don't have a gateway in your network.

DHCP and DHCP relay
There are several methods for delivering boot information to the hosted system. The default method of delivering IP and storage information to boot Windows uses an integrated Dynamic Host Configuration Protocol (DHCP) server on i5/OS side of the iSCSI network. Even with DHCP, the IP address may be considered static because the DHCP server associates a single IP address with a MAC address. For more information, see Diskless booting over iSCSI.

The integrated DHCP server is designed to coexist with any DHCP servers that might also be on the iSCSI network.

If the iSCSI networks includes routers between the iSeries™ server and the hosted system, and the boot information delivery method is DHCP, then an appropriately configured DHCP relay agent, also known as a BOOTP relay agent, is required in the network.

Performance and maximum transmission unit (MTU)
iSCSI HBAs can use either of the following as a maximum transmission unit (MTU) value:

1500 bytes. This is the standard size for Ethernet frames and is the default value in the iSCSI HBA. If you configure an iSCSI HBA to use an MTU of 1500, you do not need to be concerned about Ethernet switch MTU compatibility.
9000 bytes. An industry term for frames larger than 1500 bytes is 'jumbo frames'. If you configure an iSCSI HBA to use an MTU of 9000, ensure that all network equipment involved, such as switches, is capable of handling an MTU of at least 9000 and is configured to do so. If this condition is not satisfied, the xSeries or blade may fail to boot (typically with only a blinking cursor on the xSeries or blade console).

The MTU setting can affect performance. The MTU value that provides the best overall performance in your environment depends on several factors, including your switch characteristics, your applications, and your CPU resources.

Of the gigabit Ethernet switches that support an MTU of 9000, some cannot sustain full wire speed with an MTU of 9000 but can with an MTU of 1500. With these switches under heavy traffic, it is possible that using an MTU of 9000 can decrease the performance of both storage and virtual Ethernet. If you are not sure that your switch performs well with an MTU of 9000, you may want to use the standard MTU of 1500.
As long as switch limitations are not affecting performance, setting the iSCSI HBA and switch MTU configuration to 9000 typically improves performance. Virtual Ethernet performance improves more than storage performance. This is because storage uses TCP/IP hardware in the iSCSI HBAs to process Ethernet frames, while virtual Ethernet uses the System i CPU to process Ethernet frames. For large data transfers over virtual Ethernet, using an MTU of 9000 instead of 1500 reduces System i CPU utilization because there are fewer frames to process.

For information about configuring MTU, see Maximum transmission unit (MTU) considerations

Managing i5/OS iSCSI adapter utilization
Paths configured in the network server description control what storage traffic, if any, and what virtual Ethernet traffic, if any, can flow over an i5/OS iSCSI adapter. For more information, see Manage iSCSI HBA usage.

Multiple hosted systems can use an i5/OS iSCSI adapter simultaneously if multiple network server descriptions use the same network server host adapter object.

Managing hosted system iSCSI adapter utilization
You can configure an iSCSI adapter in a hosted system with a SCSI IP address, a LAN IP address, or both kinds of IP addresses. The presence of a SCSI IP address enables storage traffic, and the presence of a LAN IP address enables virtual Ethernet traffic. Each Windows virtual Ethernet adapter is normally automatically assigned to a physical iSCSI adapter. There is an option on the advanced properties tab of each virtual Ethernet adapter that allows a particular physical iSCSI adapter to be selected. See Manage iSCSI HBA allocation at the Windows side of the iSCSI network.

IBM® does not support the use of the iSCSI adapter as a general purpose external network connection. For more information on external network connections, see External networks.

Other considerations

The iSCSI network only uses Internet Protocol version 4.
The frame format is Ethernet version 2.
The iSCSI network does not support Network Address Translation.

Security
There are several ways to secure storage and virtual Ethernet traffic. For more information, see Security concepts.