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This design guide on networkcomputing.com states the following:

When uplinking to Nexus 5Ks, one topology option is to dual-home the physical Fabric Extender. Dual-homing a FEX provides path redundancy, but cuts in half the total number of Fabric Extenders you can deploy

In the following Cisco Data Sheet it states that "Maximum Fabric Extenders dual-homed to a vPC Cisco Nexus 5000 Series or Nexus 5500 Series switch pair" for a Nexus 5000 series switch is 12. It also states "Maximum Fabric Extenders per Cisco Nexus 5000 Series or Nexus 5500 Series switch" is the same, 12. This doesn't make sense according to the networkcomputing.com guide. Dual homed FEXs connected to Nexus 5000 vPC should be half the supported amount of a single-homed FEX design.

Let's say I have two Nexus 5000 series switches. They support a total of 24 FEXs if each FEX is dual homed. If I configure vPC on my 5000 series switches and dual home my FEX up to each switch, will I now only be able to connect 12 FEXs instead of 24 using these two switches?

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From the Cisco NX-OS 5.1(3)N1(1) release and later releases, each Cisco Nexus 5500 Series device can manage and support up to 24 FEXs without Layer 3. With Layer 3, the number of FEXs supported per Cisco Nexus 5500 Series device is 8. With Enhanced vPC and a dual-homed FEX topology each FEX is managed by both Cisco Nexus 5000 Series devices. As a result, one pair of Cisco Nexus 5500 Series devices can support up to 24 FEXs and 16 FEXs for Layer 2 and Layer 3.
There are differences in scalability between the straight-through topology, the dual-homed FEX topology, and the Enhanced vPC topology. In the straight through topology, only one Cisco Nexus 5000 Series device manages each FEX and a pair of Cisco Nexus 5500 Series devices manage up to 48 FEXs

enter image description here

Because the total number of FEXs that are supported by a pair of Cisco Nexus 5000 Series devices is different between these two topologies, the FEX straight-through design with more than 24 FEXs per one pair of Cisco Nexus 5000 Series devices cannot migrate to Enhanced vPC topology.

The configurations are as follows:

•Total number of host vPC — With Enhanced vPC, each FEX port can be part of a host vPC. The host vPC does not consume port channel resources on the parent Cisco Nexus 5000 Series device.

•Total number of ports per host vPC — The total number of ports that can be assigned to each host vPC differs with each FEX model.

please may you check this link Using Enhanced vPC

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You need to read the fine print in the Cisco documentation:

Maximum Limits—Indicates the maximum scale capability tested for the corresponding feature individually. This number is the absolute maximum currently supported by Cisco NX-OS Release 5.0(3)N1(1b) software for the corresponding feature. If the hardware is capable of a higher scale, future software releases may increase this maximum limit.

Just because you have the hardware capability to connect a lot of devices doesn't mean that the software will support it. Based on the note above, the number that the software supports may vary per code release, and it may be lower than the number of physical connections you can make.

It seems pretty clear from the table that the number of FEX on a 5000 or a 5000 vPC pair is the same number:

enter image description here

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  • I don't think you understood my question. Perhaps you should read it again. My question is: will a dual-homed FEX count as two total FEXs (a single FEX for each 5500) toward the software limit of number of FEXs supported on a Nexus 5000 switch. Your answer "just because the hardware can support more doesn't mean there isn't a software limitation" is completely irrelevant – Michael May Sep 7 '15 at 23:10
  • I think you are reading too much into it. The 5000, with this software version, can support 12 FEX on a single switch, or, as a vPC pair, it can support 12 FEX. It is a software limitation. I don't know why you would think a single FEX counts as two to the software. A dual-homed FEX counts as two to the hardware (needs two physical ports). – Ron Maupin Sep 7 '15 at 23:39
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    The FEX capacity of N5k switches has changed with software releases, in this code version the N5000 can only support 12 FEX's, either single- or dual-attached (the N5500 gets 24). The dual-attached FEX's don't count as 'two', but you only get 12 dual-attached FEX per vPC pair, instead of 12 single-attached FEX per vPC member (which makes 24 total). If you would provide the exact hardware and software you are working with, we could provide an exact answer but at this time we don't know if you have a first-gen (N5000) or second-gen (N5500) switch. – cpt_fink Sep 8 '15 at 4:22

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