We have a pair of 6500 switches, with two 20G uplink to a pair of stacked 3750E switches, on the 3750E stack, one 20G is in forwarding state, while the other 20G link is in block state. Since we want to make sure that when one of the 10G link is down within the active 20G uplink, the STP will block the 10G port-channel on the 3750E stack, and put the 20G port-channel to forwarding, I have used STP long form, and it works perfectly.
But there is new requirment that I need to connect the two 6500 switches to another pair of 6500 switches from another department. We create virtual interface on our pair of 6500 switches, configured a new VLAN for this new connection, and run HSRP. The other pair of 6500, which are not supported by us, create the same VLAN, have a pair of FWSM, which connect to the same VLAN of our 6500 switches. Our 6500 switches have fiber connect directlyl to the two new 6500 switches. So basically, there is a VLAN common to both pair of 6500 switches (and hence using STP).
The problem is, since this new pair of 6500 switches are running STP short method, and can't change to STP long method to match our setting, my question is, will there be problem when two pair of 6500 switches, running different STP method (one pair running short-method, another running long-method).
If it is not desirable, i would need to change my STP method back to short method, and find another solution to fix my 20G to 10G STP issue. Any suggestion here?