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CCIE - MST Question: Port costs with pre-standard devices...

Hi People!

Just studying and I have a question about MST link costs which I have noticed, whereby devices that only support pre-standard MST seem to exist in a MST domain all of their own...

Take the following scenario, all lines are trunks forwarding all VLANs; the lab was setup from a write erase / blank vlan.dat state...


The 2950s support Cisco pre-standards MST. The 3750X support standards based MST. Latest IOS is installed on all devices, and have been rebooted from a cold start...

Assuming the MAC addresses above have been simlified to show the order, in my opinion, the root bridge should be device 1 with the lowest MAC address (with all designated ports), and a root port on devices 2, 3 and 4 towards device 1. Nothing wrong with this on the face of it from my limited understanding (not including other port states for blocking links)...

In a lab I have put together the above network and that is not what I am seeing!

Device 3 and 4 are seeing devices 1 and 2 as pre-standards MST (not a problem in it's self):

  • The link costs between and to the pre-standards devices (2950 destined links) are all 200,000.
  • The link cost between the two standards based devices (3750X) are 20,000. (Cheaper cost than the pre-standards links)

The spanning tree table for MST 0 is as follows (red line goes from root port to designated port or arrow head):


It seems that the standards based MST capable devices are creating a little MST of their own, and working out the best point to hand off to the pre-standards tree? Also why has device 2 been chosen rather than the root bridge device 1??

If I apply all interfaces on the devices 3 & 4 (3750X) to be configured as pre-standard based ports "(config-if) spanning-tree mst pre-standards" the following behaviour is seen:


Now the above example does more along the line of what I would expect if I had two MST domains. Device 4 is showing the link to device 1 as an alternative; it is choosing the 20k cost link to device 3 over the 200k cost to device 1...

Why is this behaviour being seen?

Can anybody explain this please, or point me in the direction of some learning paths please!!

Thanks in advance...


Everyone's tags (4)
Community Member

CCIE - MST Question: Port costs with pre-standard devices...

Hi All,

On getting a cup of tea and settling back down to my rig, noticed that Devices 3 and 4 weren't trunking.  Once I enabled trunking, the behaviour I expected occured.

However, this confuses me even more:

MST 0 only has VLAN 1 (the native VLAN on the lab network) in it...

Any comments (other than the obvious user capability comments) would be most welcome!


Cisco Employee

CCIE - MST Question: Port costs with pre-standard devices...


Are you sure the 3750 were not trunking? The 2950 default to dynamic desirable so any automatic port on the other side will come up as trunk.

In addition, can you please tell which exact links were not trunking before if they indeed operated as access ports?

Best regards,


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