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STP short and long form mix

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?



Giuseppe Larosa
Hall of Fame Master

Hello Ben,

being the other switches administered by another group I would use two point-to-point routed links with no STP running over it.

Then you can have a vlan connecting the switches on your side and they can do the same on their side.

Hope to help



Thanks for your reply.

Since we are connecting to a pair of fwsm (in active/standby mode) on a pair of Cat6500, the firewall interface facing us needs to be in the same subnet,and so we can't do a point to point direct connection to the two fwsm interfaces. We did think about using layer 3 interface on our Cat6500, then connect to their Cat6500 vlan, but in this case, anything happen to their Cat6500 interlink (the link between their Cat6500), will make both our Cat6500 HSRP to be active, that would not be good.



Hi Benny,

This will worek. STP always encode the cost on 4 bytes in the BPDUs, whether you are running short or long.

So if you are running short and advertise a cost of X, the switch in long mode will receive a value Y. That's not a big deal and you can still influence the values by doing cost tuning on the interfaces.

Just play around with the setup. I don't mean it's going to be straightforward to implement or pretty, but it should not break anything.



Hi Francois,

Thanks for your reply.

Just to clarify what you mean. So you said that mixing STP short and long method in the same network is okay? I am asking because from Cisco document, it advise to change all switches to the same method.

Is there other better design to solve this issue beside mixing short and long method?



Hi Francois,

Also, is there any Cisco documentation or webpages that talks about the mixing of STP short and long method?



The only mention of this you will find is: don't do it;-)

My point is: it's not impossible, it's just painful.