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?
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
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