Migrating STP Root and HSRP Active/standby routers

[N3t W0rK3r]

We are in the midst of migrating our core infrastructure from four Cat6500s to two Nexus 7Ks with 5Ks.

One of the tasks I am charged with is migrating that STP roots for all of our VLANS from our Cat to our Nexus7K, in addition to migrating the hsrp active & standby routers for all of our vlan interfaces from the 4 Cats to the two Nexus 7Ks.

I'm looking to this community for a bit of guidance and some suggestions on how best to accomplish this, with minimal interruption to service, as we are a large, regional healthcare provider.

It should be noted that we have a large number of vlans (and vlan interfaces) in use that must be moved.  To date, we have implemented RSTP on the Nexus side of our infrastructure, but the Cat side (where the STP root is for most VLANS) still uses PVST+.  Currently, most of our edge devices now connect physically to the Nexus side of our network.  However, most of our servers (on a few vlans) are still connected to the Cats.

Some specific questions I have are:

1. How long of a network interruption should I expect on a VLAN when moving that VLANs STP primary root and secondary root to my N7Ks?
2. Same question as 1. above but for the HSRP vlan interface?
3. Is there a specific order that I should move these for each vlan?  ie. Move the hsrp then stp root, or vice versa?
4. Should I be using the vpc peer-switch feature on my N7K vpc domain?  I currently do not have that configured, so what will be the impact to the network at the time I do implement it?  Should I implement before or after moving all the stp roots?

Any suggestions are greatly appreciated.

Thank you in advance.

John

[marce1000]

 - Normally it shouldn't be too difficult to do this; as per STP root makes  sure that the VPC

nex-peer becomes the second  in prio , after the current highest (the cat...) ; same for hsrp; configure

the nex-pair as 3th in priority so that they are defaulting to primary once the catxxx is turned off.

Vlan's should be defined 'from the cat6pair' and active; then the switch to vtp master can also be easily made.  This is theoretically. 

Personally I would prefer to this with help from my cisco partner or reseller as unknown gotcha's can still happen, especially I would do it this way if your environment is 'production-critical'.

M.