MSTP issue with Cisco switch

Good morning everybody,

After months of passive reading, the time has come for my first forum post.

To simplify my topology, I have an F5 3600 equipped with TMOS 10.2.4-build577, connected to a Cisco 2960 switch with two dot1q links: the former (VLAN 603) communicates with the public firewall, the latter (VLAN 600) with the private firewall.

I need Spanning tree because, actually, there are two LTM appliances in Active/Passive mode connected to the same switch stack.

Both F5's suffer the very same condition.

I previously tried with RSTP, but switched to MSTP hoping that separated instances would help.

On the surface, the second cable is blocking.

Some data might help:

- F5:

root@F5(Standby)(tmos) list net stp-globals
net stp-globals {
    config-name MSTP-PFQ-PUB
    config-revision 1
    mode mstp
}


root@F5(Standby)(tmos) show running-config net stp
net stp 0 {
    priority 49152
}
net stp 1 {
    interfaces {
        1.5 {
            external-path-cost 20000
            internal-path-cost 20000
        }
    }
    priority 49152
    vlans {
        600
    }
}
net stp 2 {
    interfaces {
        1.7 {
            external-path-cost 20000
            internal-path-cost 20000
        }
    }
    priority 49152
    vlans {
        603
    }
}
 
[root@F5:Standby] config  bigpipe stp
STP   MODE mstp
|     Forward delay 15   Hello time 2   Max age 20   Transmit hold 6
|     Max hops 20   Revision 1   ID MSTP-PFQ-PUB
+-> STP INSTANCE 0   priority 49152   root bridge 04:DA:D2:CC:B0:00
|   |     regional root bridge 00:01:D7:BE:E5:40
|   |     No topology changes
none+-> STP INSTANCE 1   priority 49152   regional root bridge 00:01:D7:BE:E5:40
|   |     No topology changes
|   +-> STP VLAN 1/Int_Interco_Pub
|       +-> STP INTERFACE 1/1.5
|           |     path cost 20000   priority 128   role master
|           |     state forward (forward)   link p2p   not edge - auto
+-> STP INSTANCE 2   priority 49152   regional root bridge 00:01:D7:BE:E5:40
    |     No topology changes
    +-> STP VLAN 2/Ext_Interco_Pub3
        +-> STP INTERFACE 2/1.7
            |     path cost 20000   priority 128   role alternate
            |     state block (block)   link p2p   not edge - auto

Switchshow version
Cisco IOS Software, C2960S Software (C2960S-UNIVERSALK9-M), Version 12.2(55)SE5, RELEASE SOFTWARE (fc1)
 
Switchshow spanning-tree mst configuration
Name      [MSTP-PFQ-PUB]
Revision  1     Instances configured 3

Instance  Vlans mapped
--------  ---------------------------------------------------------------------
0         1-400,402-510,512-599,601-602,604-4094
1         401,511,600
2         603
-------------------------------------------------------------------------------

Switchshow spanning-tree vlan 600

MST1
  Spanning tree enabled protocol mstp
  Root ID    Priority    1
             Address     04da.d2cc.b000
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    1      (priority 0 sys-id-ext 1)
             Address     04da.d2cc.b000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/1             Desg FWD 20000     128.1    P2p
Po3                 Desg FWD 20000     128.240  P2p
Gi2/0/2             Desg FWD 20000     128.56   P2p

Note: g1/0/1 is connected to F5 n.1, g2/0/2 to F5 n. 2 and po3 to the private firewall


Switchshow spanning-tree vlan 603

MST2
  Spanning tree enabled protocol mstp
  Root ID    Priority    2
             Address     04da.d2cc.b000
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    2      (priority 0 sys-id-ext 2)
             Address     04da.d2cc.b000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/7             Desg FWD 20000     128.7    P2p
Po5                 Desg FWD 20000     128.256  P2p
Po6                 Desg FWD 20000     128.264  P2p
Gi2/0/8             Desg FWD 20000     128.62   P2p
 
Note: g1/0/7 is connected to F5 n. 1, g2/0/8 to F5 n. 2 and po5-6 to the public firewall.

Maybe I misconfigured anything?