why vPC+ in fabric path

vPC and vPC+ at edge, understanding the differences

vPC is a technology that is around for a long time now and is a very intelligent (MLAG) solution for the nexus product line.
With Cisco FabricPath's introduction vPC+ was also introduced.
lets see how different vPC+ is and how vPC+ overcomes some problems that vPC has in a fabricpath environment.

Below is a diagram where standard vPC is configured in a fabricpath environment.

Server A with MAC (aaa.aaa.aaa) is connected to nexus 5k-1 and 5k-2 in a vPC.

so lets take it step wise:

1. Server A sends traffic to Server B
2. Server A can use either of the links in vPC depending on the hashing
3. lets say Server A takes outgoing path via 5k-1 (using switch ID of 5k-1)
4. 5k-1 sends traffic to 7k-4 via fabricpath
5. 7k-4 sends traffic to 5k-3 and finally Server B receives the frame as classical ethernet

now the problem arises in the reverse path

5k-3 has to send traffic back to Server A , so MAC(aaa.aaa.aaa) is behind which Switch ID. 5k-1 or 5k-2 ??? so it send back to 5k-1 and we do not achieve multipathing

 This problem is resolved by using vPC+

Below is an example of vPC+ with Fabric path domain

So lets take it step wise:

1. Server A sends traffic to Server B with source MAC(aaa.aaa.aaa)
2. Server A uses either links in vPC+ depending on hashing
3. say 5k-1 receives the frame , it encapsulates the frame in cisco Fabric path header with source switch-id 12(emulated switch ID)
4. 5k-1 forwards the frame to 5k-3 via 7k-4 (basically the best fabric path SPT)
5. 5k-3 forwards the frame as classical Ethernet to the host server B

So now in reverse path the frame from 5k-3 will be sent to emulated switch ID-12 which is physically either 5k-1 or 5k-2 , thus achieving true Multipathing

So now lets talk about the benefits of using vPC+ at edge

Functions that are enabled by vPC+ at the edge include:

• attach servers to edge switches with port-channel teaming

• attach additional Classic Ethernet switches in vPC mode

• attach Cisco Fabric Extenders in FEX Active/Active mode.