Mainframe OSAs and trigger BPDU guard and error disables switch port

When you upgrade microcode on IBM OSAs when they come back up they may act as a 'bridge' and send BPDUs to your switch which fires bpudguard. You do not want to change the switch. what you have done is have the mainframe people stop and start the OSA, make sure the UCB is not 'boxed', and you should be fine.

NX-OS changes how type 5 external routes work

NX-OS supports and rfc 2328 which says that if there are two paths to an ASBR one through area 0 and one that is not the NON AREA 0 is what is preferred. So when you move to nexus devices or add new routers, two type 5 external routes may not be put in as equal cost routes even though all the costs are equal. So you may need to add a non 0 area to your new router to get the load balancing you want. There are other factors to be considered in complex topologies see http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/117824-config-ospf-00.html

You do not get a default route in a stub area without an ABR (link in A 0)

I use Vrf Lite a lot and there you use stub areas, sometimes I have a local segment (The VRF is on a single distribution pair) and then need to expand it, or I am building a VRF that statically routes to a firewall. To get a default route, that has to come from an ABR, so some interface has to be in Area 0. If you are statically routing to a firewall make the interfaces routing to that firewall area 0 interfaces and then the switch that has the firewall connection sends out a default route to any other routers in the VRF