Data Center Design Recommendations

[S891]

Hi, 

I have the following design under consideration. I have two DCs that are not far apart. The Aggregation layer for the data centers is on Layer 2 Nexus 9500 and Nexus 9300 respectively. I am planning to use a Catalyst pair for combined Layer 3 of the two DCs. There will be services layer attached to the layer 3 catalysts (FW, LB etc.). There are VLANs that span across both DCs. 

Here are my questions:

-Is it better to setup the HSRP Primary and STP root on the same Catalyst Pri Switch as it is recommended to configure the same switch for both features? DC best practices guidelines suggest to use vPC Pair for the root of STP so here is a cofusion. 

- Should I manually adjust STP port priority to influence the forwarding traffic such as keep the Dual sided vPC in the blocking state or remove these links completely? The reason I want to actually keep these links is because I want intra-vlan traffic to go directly between the two DCs rather than traversing the extra hop (catalyst). 

- Does the vPC member link go into blocking state? In this setup, would the double-sided vPC will go in blocking state in any given circumstances?

- Currently the layer 3 resides on Pri DC Nexus 9500. When I am adding the Catalyst pair I want to keep layer 3 on both Nexus 9500 and Catalyst. Is it possible to do layer3 over vPC member link or should I consider a separate link? 

- Any other suggestion/recommendation will be appreciated

[S891]

I think that the double-sided vPC between the two DCs will not be in forwarding state but I am not sure if there is any other benefit for keeping it. 

[Alex.M]

Hi Fawad-Alam,

Reading through your requirements and concerns, it sounds like the catalyst will be the edge for layer 3 connections and will not need layer 2 past this point.

Instead of redesigning your layer 2 to 3 boundary, have you considered making the interfaces towards the catalyst switch layer 3?  In this type of design, you will maximize your bandwidth utilization from half to full with ECMP, your FHRP does not need to be moved, and STP does not become a problem.  This would nullify all your questions about STP, HSRP, vPC blocking, etc.  Keep in mind, that vPC is strictly a layer 2 protocol and is (for the most part) not layer 3 aware.

HTH,

Alex