I found some documentation that I wrote a few years ago on the Nexus product line, a few things have changed since I wrote this, but it is largely true.
Dual Homed or Active/Active Fex Design
The Nexus 7K to 2K does not support dual homed connectivity. Cisco does support this in 5K to 2K design, but not in 7K to 2K. The single homed design is the only supportable solution. This has implications that if there is a single Nexus 7K failure, it will take down all of it’s down stream FEX’s. Cisco stance is that the probability of a Nexus 7K going down with redundant sup’s and power configuration is less likely then two 5Ks going down.
Lack of support for multi-speed connectivity
The current line of Nexus 2K’s and 5K’s only support either 10/100/1000 Mbps or 1/10 Gbps, but not both. This leaves the pod to support only one or the other. While all production traffic is expected to support 1 Gbps, often times the out-of-band connections do not have 1Gbps connectivity. For these situations, cables will be ran back to the Core-A and Core-B pods.
VDC is limited to entire FEX
Since FEX’s are considered to be simply line card extenders, the hypothesis could be made that a each port can be applied to various VDC’s, however this is not the case. An entire FEX can only be applied to a given VDC, not a member of multiple VDC’s. As such this requires separate devices for multiple ToR switches for multi-tenant environments. The premise of having ports on a given FEX separated per VDC is not currently on Cisco’s road map.
Only 4 VDC’s allowed (since changed to 16 based on SUP)
A given Nexus 7K can only support 4 VDC’s, this is currently only a software limitations, which should be extended in future releases. This actually only gives you 3 unique VDC’s since one must be dedicated to the Management of the device.
Allocation of ports to VDC’s on 7K
Though mainly trivial, it is of note how VDC’s can be divided in a given line card. They are not consecutive assignments, instead, in a given set of 8 ports the odds and evens have to all be allocated.
BPDU’s on 2K’s
The Nexus 2K’s have spanning-tree BDPU guard enabled by default, and cannot be turned off. The implications are, if you have a downstream switch, or device that supports BPDU’s must shut of the BPDU.
How many FEX’s supported per Device
32 Fex’s per 7K and 24 per 5K. This does not take into consideration that when a device is VPC’d it counts against both head end devices.
Backplane is synonymous with uplink speed
Since all traffic must traverse the uplinks, there is no backplane per sey. It is essentially just the size of it’s uplinks, so if you want a bigger backplane, you must expand the uplinks.
Multi-tiered sets of FEX’s
You cannot fex a fex, meaning there is no ability to have a FEX downstream from a FEX.
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