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Source: Microsoft Support
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If you start a Windows Server 2003, Windows Server 2008, or Windows Server 2012 R2 system using iSCSI Boot with the Microsoft iSCSI Boot Initiator, the gateway settings specified in the iSCSI Boot solution will always be used by Windows to reach the iSCSI Target hosting the boot drive, even if the iSCSI Target is directly reachable (on-link) and is not on a network disjoint from the iSCSI Boot NIC.
While starting, the Microsoft iSCSI Boot Initiator creates a static route to the iSCSI Target which contains the gateway specified in the iSCSI Boot solution. This route is a system-critical route that cannot be removed.
Because the gateway from the iSCSI Boot solution is specified in the route, the gateway will always be used in communication with the iSCSI Target, even if the iSCSI Target is directly reachable (on-link) and a gateway is not required to communicate with the iSCSI Target.
In an iSCSI Boot environment, the optimal configuration is to have a NIC dedicated to iSCSI traffic and a separate NIC or NICs used for network communication with other servers or workstations. In an iSCSI Boot environment, the iSCSI Boot NIC being used to communicate with the iSCSI Target should only be used for communication with the iSCSI Target.
The NIC being used to communicate with the iSCSI Target should be configured to communicate with the iSCSI Target using the most efficient network route possible. For example, if a gateway is not needed to reach the iSCSI Target, then one should not be specified in the iSCSI Boot solution. This will prevent network traffic to the iSCSI Target from being unnecessarily routed through a gateway.
The Default Gateway specified in the TCP/IP settings for the iSCSI Boot NIC is separate from the gateway specified in the static route created by the Microsoft iSCSI Boot Initiator. The Default Gateway is used to communicate with devices on disjoint networks. The gateway specified in the static route created by the Microsoft iSCSI Boot Initiator will take precedence over any Default Gateway specified in the TCP/IP settings for the iSCSI Boot NIC. For more information Default Gateways, please review the following KB article:
157025 Default Gateway Configuration for Multihomed Computers
Multiple Default Gateways should not be specified on a system, as you may experience connectivity problems. For more information regarding multiple Default Gateways, please review the following KB article:
159168 Multiple Default Gateways Can Cause Connectivity Problems
The static route is created by the Microsoft iSCSI Boot Initiator to ensure communication with the iSCSI Target is maintained at all times, as the iSCSI Target contains files critical to the operation of Windows.
The gateway settings for the static route are passed from the iSCSI Boot solution to the Microsoft iSCSI Boot Initiator by way of the iSCSI Boot Firmware Table (iBFT). For more information on the function and purpose of the iBFT, please review the following KB article:
953528 You may experience unpredictable results in Windows Server 2003 when you start the operating system from iSCSI
To view the active routes on a system, including the system-critical route established when using iSCSI Boot, use the 'route print' command from the Command Prompt.
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