We use a Catalyst 6500 VSS System as CORE and Nexus 5010 in the Access Layer. (no aggregation layer). So both System can made Port-Channel spanned over two Chassis.
This is the actual Design. Each link is 10G. The speciality is, we only use one Uplink per N5k because we don't need more Bandwith at the moment and we have the redundancy. The VSS System is the STP Root Bridge.
Problem: Reloading Nx1 causes Nx2 to Block the wrong Port when the Nx1 comes up. The Nx2 Spanning Tree bridge assurance blocks the Port marked with a red 'X' for about 30 seconds. This is a real Problem because the Nx1 is not fully ready to this time and so the Systems loose connectivity for 30 seconds. Too long!
The Problem is _not_ to the time when Nx1 is shut down, it is when Nx1 has finished booting and takes the Switchports up.
: 2010 Apr 29 15:40:36 CET: %STP-2-BRIDGE_ASSURANCE_BLOCK: Bridge Assurance blocking port port-channel1 VLAN0109.
: 2010 Apr 29 15:41:11 CET: %STP-2-BRIDGE_ASSURANCE_UNBLOCK: Bridge Assurance unblocking port port-channel1 VLAN0009.