Chapter 4. IBM System Storage DS3500 and EXP3500 Cabling 97
General guidelines when using Fibre Channel cables
You should always follow these general guidelines when using Fibre Channel cables:
򐂰 Do not route the cable along a folding cable-management arm.
򐂰 For devices on slide rails, leave enough slack in the cables so they do not bend to a
diameter of less than 76 mm (3 in.), or a radius less than 38 mm (1.5 in.) when extended
or become pinched when retracted.
򐂰 Route the cable away from places where it can be damaged by other devices in the rack
cabinet.
򐂰 Do not use plastic cable ties in place of the provided cable straps.
򐂰 Do not overtighten the cable straps or bend the cables to a diameter of less than 76 mm (3
in.), or a radius less than 38 mm (1.5 in.).
򐂰 Do not put excess weight on the cable at the connection point. Be sure that the cable is
well supported.
򐂰 The following are the recommended maximum cable lengths.
2 Gbps: 300 m (984 ft.) of 50/125 um fiber, 150 m (492 ft.) of 62.5/125 um fiber
4 Gbps: 150 m (492 ft.) of 50/125 um fiber, 70 m (230 ft.) of 62.5/125 um fiber
8 Gbps: 50 m (164 ft.) of 50/125 um fiber, 21 m (115 ft.) of 62.5/125 um fiber
4.4.3 Interoperability of 2 Gbps, 4 Gbps, and 8 Gbps devices
The Fibre Channel standard specifies a procedure for speedy auto-detection. For example, if
a 4 Gbps port on a switch or device is connected to a 2 Gbps port, it must negotiate down and
the link will run at 2 Gbps. If there are two 8 Gbps ports on either end of a link, the negotiation
runs the link at 8 Gbps if the link is up to specifications. A link that is too long or “dirty” can end
up running at 4 Gbps, even with 8 Gbps ports at either end, so care must be taken with cable
lengths and that connector quality is sound.
The same rules apply to 8 Gbps devices relative to 4 Gbps and 2 Gbps environments. The
8 Gbps and 4 Gbps devices have the ability to automatically negotiate back down to either
4 Gbps, 2 Gbps, or 1 Gbps depending on the attached device and the link quality. If the link
does unexpectedly negotiate to a slower speed than expected, then the causes or reasons for
this ought to be investigated and remedied.
The DS3500 Storage System has 8 Gbps functionality: there are several SAN switches,
directors, and Host Bus Adapters that can operate at this speed.
4.5 iSCSI Ethernet cables
In this section we describe the iSCSI Ethernet cabling that you use for iSCSI connections to
hosts.
Note: Refer to Table 3-1 on page 39 for the more information about the supported
distances for the OM1 and OM2 category Fibre Channel cables.
Note: On certain Fibre Channel switch vendor models, it might be necessary to manually
configure the port to a specific speed of 2, 4, or 8 Gbps to obtain the required speed
instead of leaving “auto-detection” on the port.

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