Simple MST setup - config and some questions

[Maurice David Wörnhard]

Hi all

(I also cross-posted to the new forums at http://h30499.www3.hp.com/t5/Legacy/bd-p/switching-legacy-forum)

Over the weekend I transitioned from RSTP to MST in our pretty simple network, but I wonder if I have done it right.

I am using one 3500yl and 5 3400cl. The topology is shown on the attached PNG...


All switches are in the same region and have the same mst configuration digest.

(all switches)# spanning-tree instance 1 vlan 1 2
(all switches)# spanning-tree instance 2 vlan 3 4

In order to have some load-balancing between the two fiber trunks MST was set up like this:

sw-1 and sw-3:
# spanning-tree instance 1 trk1 path-cost 6667
# spanning-tree instance 1 priority 2
# spanning-tree instance 2 trk1 path-cost 20000
# spanning-tree instance 2 priority 4

sw-2 and sw-4:
# spanning-tree instance 1 trk1 path-cost 20000
# spanning-tree instance 1 priority 4
# spanning-tree instance 2 trk1 path-cost 6667
# spanning-tree instance 2 priority 2

No special modifications on sw-5 and sw-6.

I also would like sw-3 or sw-4 to be both regional and IST roots, so I increase their priority:

sw-3# spanning-tree priority 0
sw-4# spanning-tree priority 1

Does this look correct? The network works as expected so far, and

show spanning-tree root-history ...

indicates that the correct switches are roots.

Some questions:
- What does "spanning-tree path-cost/priority nnn" do when using MST? Are these values just used for CST?
- Any suggestions on how to check that the VLAN-load-balancing actually works as expected?

Best
Maurice

[Michael_Breuer · Accepted solution]

Hello Maurice,

some comments: You should assign unique prio values to each switch - i.e. prio 1 for sw-1, prio 2 for sw-3 and so on. otherwise the MAC address will decide which switch will be root.
But everything else is looking fine.

spanning-tree path-cost/priority nnn will assign path cost for instance 0 (IST).

Loadbalancing can be checked with:
# show int port-utilization

Cheers,

Michael

[Maurice David Wörnhard]

Thanks for the clarifications, Michael!
/Maurice