Nexus Data Center Design Question

[Craddockc]

Community,

Im still pretty new to the Nexus line of products as well as proper Data Center design best practices using Nexus products. I did have a question regarding the thought process behind decoupling the Nexus chassis, moving away from VSS and toward vPC. What is the overwhelming benefit to allowing the control planes on each Nexus device to operate independently, versus say a StackWise or VSS implementation where the control planes are combined? Why was this decision made? Does it have to do with the "East-West" nature of the traffic? I can see the benefit of having each vPC peer having independent control plane functionality in case one system decides to misbehave, but its still unclear to me the benefits and drawbacks of decoupling them vs combining them in VSS.

Any thoughts are welcomed, thanks for your time!

[Manish Gogna]

Each peer device in the vPC domain runs its own control plane, and both devices work independently. Any potential control plane issues stay local to the peer device and does not propagate or impact the other peer device. From a Spanning-Tree standpoint, vPC eliminates STP blocked ports and uses all available uplink bandwidth. Spanning-Tree is used as a fail safe mechanism and does not dictate L2 path for vPC-attached devices.

We eliminate the Spanning-Tree blocking ports but spanning-Tree still running in the background. While still operating with two separate control planes, vPC helps ensure that the neighboring devices connected in vPC mode see the vPC peers as a single spanning-tree and LACP entity. For this to happen, the system has to perform IEEE 802.3ad control-plane operations in a slightly modified way (which is not noticeable to the neighbor switch). Please refer to the followign design docuemnt for more details

http://www.cisco.com/en/US/prod/collateral/switches/ps9441/ps9670/design_guide_c07-625857.pdf

HTH

Manish

[Craddockc]

Manish,

Thanks so much for your reply. I apologize for not responding sooner. This makes alot of sense. It makes sense that the spanning tree blocking port would be the main antagonist to being able to separate the control planes of each device. It seems the main motivation for separating the control planes is for fault tolerance should one of them misbehave. Thanks again for your reply.