[c-nsp] STP over Port-channel issue

[james list]

dear experts
a customer of mine has a legacy environment with 4 x Cisco 9500 (IOS XE
17.09.03) connected in a square mode with 2 links (2 per each connection)
and each couple of links is considered a single virtual port (port-channel).
Loops are managed with PVSTP.
Two x C9500 are in DC1 while the other two x 9500 are in DC2.

9500-1 == 9500-3
   ||                 ||
9500-2 == 9500-4

The issue is: when there is a provider flapping, ie one out of the two link
connecting C9500 in the two DC is flapping, we see one port flaps,
port-channel remains UP but PVSTP reconverge and there is a communication
failure.

The question: since the PO remains up, why we see this behaviour ?
are BDPU sent just over one link (ie the higher interfac e) ?
how can we solve this issue keeping this scenario ?
moving to RSTP could solve ?

Thanks in advance
Cheers
James

here some logs:

May  6 11:48:35.590: %LINEPROTO-5-UPDOWN: Line protocol on Interface
TwentyFiveGigE1/0/48, changed state to down
May  6 11:48:36.593: %LINK-3-UPDOWN: Interface TwentyFiveGigE1/0/48,
changed state to down
May  6 11:48:40.343: %LINK-3-UPDOWN: Interface TwentyFiveGigE1/0/48,
changed state to up
May  6 11:48:44.171: %LINEPROTO-5-UPDOWN: Line protocol on Interface
TwentyFiveGigE1/0/48, changed state to up



#sh etherchannel summary
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP        Twe1/0/1(P)     Twe1/0/2(P)
12     Po12(SU)        LACP        Twe1/0/47(P)    Twe1/0/48(P)



Port-channel: Po12    (Primary Aggregator)

------------

Age of the Port-channel   = 187d:15h:09m:01s
Logical slot/port   = 9/12          Number of ports = 2
HotStandBy port = null
Port state          = Port-channel Ag-Inuse
Protocol            =   LACP
Port security       = Disabled
Fast-switchover     = disabled
Fast-switchover Dampening = disabled

Ports in the Port-channel:

Index   Load   Port        EC state        No of bits
------+------+------+------------------+-----------
  0     00     Twe1/0/47      Active             0
  0     00     Twe1/0/48      Active             0

Time since last port bundled:    0d:01h:50m:00s     Twe1/0/48
Time since last port Un-bundled: 0d:01h:50m:09s     Twe1/0/48


# sh spanning-tree interface port-channel 12 state
VLAN0015            blocking
VLAN0016            forwarding



# sh spanning-tree vlan 15 detail

 VLAN0015 is executing the rstp compatible Spanning Tree protocol
  Bridge Identifier has priority 20480, sysid 15, address 8c84.4236.5fe0
  Configured hello time 2, max age 20, forward delay 15, transmit
hold-count 6
  Current root has priority 8207, address 444c.a830.ff71
  Root port is 2089 (Port-channel1), cost of root path is 2000
  Topology change flag not set, detected flag not set
  Number of topology changes 73 last change occurred 01:52:13 ago
          from Port-channel1
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0, aging 300

 Port 2089 (Port-channel1) of VLAN0015 is root forwarding
   Port path cost 1000, Port priority 128, Port Identifier 128.2089.
   Designated root has priority 8207, address 444c.a830.ff71
   Designated bridge has priority 16399, address cc79.d760.73e0
   Designated port id is 128.2089, designated path cost 1000
   Timers: message age 15, forward delay 0, hold 0
   Number of transitions to forwarding state: 18
   Link type is point-to-point by default
   BPDU: sent 289, received 4527220

 Port 2100 (Port-channel12) of VLAN0015 is alternate blocking
   Port path cost 1000, Port priority 128, Port Identifier 128.2100.
   Designated root has priority 8207, address 444c.a830.ff71
   Designated bridge has priority 12303, address 444c.a830.fc01
   Designated port id is 128.107, designated path cost 1999
   Timers: message age 15, forward delay 0, hold 0
   Number of transitions to forwarding state: 20
   Link type is point-to-point by default
   BPDU: sent 340, received 4525304