mstp port blocking

[zulkarnain]

Hi All.

I have 2 catalyst switches running mstp and connected to core 6509 also running mstp. The connection is as below

SW1 ----- SW2

  \              /

    \          /

Core Switch MSS

The SW1 & SW2 have vlan 60,61 & 250 and Core Switch did not have vlan 60 and 61.

Port connected between SW1 and SW2 is trunk carry vlan 60, 61 and 250.

Port connected between SW1 to Core SW access port vlan 250

Port connected between SW2 to Core SW access port vlan 250

Configuration for MST in Core Switch are:

spanning-tree mode mst

spanning-tree mst configuration
name Cisco

  revision 10

   instance 1 vlan 2-4094

spanning-tree mst 1 priority 4096

Configuration for MST in both SW1 and SW2 are:

spanning-tree mode mst

spanning-tree mst configuration
name Cisco

  revision 10

   instance 1 vlan 2-4094

spanning-tree mst 1 priority 40960

Using above configuration for SW1 and SW2, the link between SW1 and SW2 become blocking. Any idea how to make the link between SW1---Core Switch or SW2---Core Switch become block?

[paul driver]

@zulkarnain wrote:

the link between SW1 and SW2 become blocking

Any idea how to make the link between SW1---Core Switch or SW2---Core Switch become block?

Why would you want to, the stp root is the 6509, both sw1/2 directly connected ports to the 6509 will be root port towards the root, (shortest preferred path towards root switch)

The link between sw1/2 one of the ports from either switch (cannot tell from your OP) will become designated and the other will be non designated (blocked)

[MHM Cisco World]

follow

[balaji.bandi]

because of the priority, you set up on the core switch. decrease switch to switch priority lower than code to switches should fix the issue.

By the way, if you sw1 to sw2 different VLAN you can set then different priority too.

also we would like to see full config all devices. to understand the better (my suggestion based on the diagram only)