Cisco UCS Supported IOM Connectivity Options

When connecting the UCS chassis to the Fabric interconnect, it’s important to follow the design rules or otherwise you may end up with unexpected behavior. UCS supports up to two fabric extenders (2100/2200 series) per chassis and two fabric interconnects (6100/6200 series) per cluster. To have a fully redundant system you will need two fabric extenders and two fabric interconnects connected as shown below the first picture (topology in the top right and top left).

Here are some UCS design rules to keep in mind:

• Direct one-to-one relationship between the FEX and fabric interconnect. Meaning each Fabric Extender (FEX) can be connected only to a single fabric interconnect. You cannot dual-home a FEX. Similarly a fabric interconnect cannot connect to more than one FEX.
• If you choose to have only one FEX in the chassis you must place that FEX into the left bay (as viewed from the rear of the enclosure)
• When using two fabric interconnects for redundancy you must establish a cluster link between them by connecting the L1/L2 ports on the first fabric interconnect to the L1/L2 pots on the second one.

Correct IOM connectivity options:

The picture below shows the supported IOM connectivity options for UCS

Incorrect IOM connectivity options:

The picture below shows some unsupported IOM connectivity options for UCS

The first topology (upper left) is not supported because the links between the fabric interconnects are missing

The second topology (lower left) is not supported because both FEX’s are uplinked to the same fabric interconnect.

The third topology (upper right) is not supported because the FEX is dual-homed to the fabric interconnects.

The fourth topology (lower right) is not supported because there is only one FEX in the chassis so that FEX should be placed into the left bay.