Introduction
What is the difference between these two vPC modes i.e. VPC and Enhanced VPC?
Solution
Enhanced vPC is a method that allows the connection of FEX in an active-active manner using vPC between the FEX and upstream Nexus switch, and at the same time connection of a device to the FEX host interfaces with a vPC.
When the FEX was originally introduced it could only be connected to a single upstream switch in what's referred to as FEX "straight through" mode. In Cisco NX-OS 4.1(3)N1 the vPC feature was introduced that allowed the FEX to be connected in an "active-active" manner i.e., dual-homed.
The problem with FEX active-active at that time was that the servers connected to the host interfaces of the FEX could not be connected using port-channels i.e., link aggregation.
What Enhanced vPC does is to remove that limitation such that we can have FEX active-active with vPC to the upstream Nexus switch, and at the same time connect hosts with port-channels.
Each of the three models is shown in the diagram.
7K supports FEX dual homing
Question: Is 7K supports FEX dual homing?
Answer: In terms of support for Enhanced vPC, that is available from NX-OS 5.1(3)N1 on the Nexus 5500 series switches and on the Nexus 6000 series switches. There is no support for Enhanced vPC on the Nexus 7000 series switch currently.
When connecting FEX to Nexus 7000 switches, active-active is not supported today i.e., up to and including NX-OS 6.2. To see what topologies are supported in the Nexus 7000, take a look at the document Nexus 7000 Fex Supported/Not Supported Topologies.
Why uplink from the 2K can not be a vPC?
Question: Also, in the second part of your post you say that the uplink from the 2K can not be a vPC? What is it supposed to be then?
Answer: As you're probably aware, virtual Port-Channel (vPC) is a method used in Cisco Nexus switches that make two switches appear to a downstream device as a single switch. When connecting a FEX to a single upstream Nexus 7000 series switch there is no vPC configuration required on the switch and the FEX is simply connected using a standard port-channel. On slide 22 of the Quick Start guide that AJ referenced (https://communities.cisco.com/docs/DOC-35725), you'll notice that there is no vpc command under the interface port-channel 199.
