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New Member

Spanning-tree question

Hi chaps,

 

I am trying to find out what's going on in a network, where I currently have loads of switches, majority of which are running RSTP, but there are still some running PVST+. I came up to this case, which surprises me a bit and I decided to ping you guys for your thoughts... Right, so here is the network:

[switch A] Gi0/1---- Gi1/0/52 [switch B] ----- [switch X] ----- [ROOT]

^RSTP^                                   ^PVST+^

 

Here is what I got on Switch A (which is running RSTP): 

SwitchA#show spanning-tree details

 VLAN0001 is executing the rstp compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 1, address 0022.be78.0d00
  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6
  Current root has priority 24577, address 001e.7a0d.91c0
  Root port is 1 (GigabitEthernet0/1), cost of root path is 14
  Topology change flag set, detected flag not set
  Number of topology changes 279036 last change occurred 00:00:21 ago
          from GigabitEthernet0/1

  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15 
  Timers: hello 0, topology change 13, notification 0, aging 300

SwitchA#show spanning-tree 

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    24577
             Address     001e.7a0d.91c0
             Cost        14
             Port        1 (GigabitEthernet0/1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0022.be78.0d00
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time  300 sec

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi0/1               Root FWD 4         128.1    P2p Peer(STP) 

SwitchA#show spanning-tree interface gigabitEthernet 0/1 detail 
 Port 1 (GigabitEthernet0/1) of VLAN0001 is root forwarding 
   Port path cost 4, Port priority 128, Port Identifier 128.1.
   Designated root has priority 24577, address 001e.7a0d.91c0
   Designated bridge has priority 32769, address 24b6.57c5.5300
   Designated port id is 128.52, designated path cost 10
   Timers: message age 15, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default, Peer is STP
   BPDU: sent 111, received 4457838

So .. it looks like I have received TCN on ROOT port, which just doesn't sound right (or I am missing something here). 

 

However, when I go on switch B (which is running PVST+) and when I check the STP details, I cannot see any TCNs being generated: 

 

SwitchB#show spanning-tree detail 

 VLAN0001 is executing the ieee compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 1, address 24b6.57c5.5300
  Configured hello time 2, max age 20, forward delay 15
  Current root has priority 24577, address 001e.7a0d.91c0
  Root port is 50 (GigabitEthernet1/0/50), cost of root path is 10
  Topology change flag not set, detected flag not set
  Number of topology changes 94 last change occurred 1w4d ago

          from GigabitEthernet1/0/52
  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

SwitchB#show spanning-tree interface gigabitEthernet 1/0/52 detail 
 Port 52 (GigabitEthernet1/0/52) of VLAN0001 is designated forwarding 
   Port path cost 4, Port priority 128, Port Identifier 128.52.
   Designated root has priority 24577, address 001e.7a0d.91c0
   Designated bridge has priority 32769, address 24b6.57c5.5300
   Designated port id is 128.52, designated path cost 10
   Timers: message age 0, forward delay 0, hold 0
   Number of transitions to forwarding state: 1
   Link type is point-to-point by default
   BPDU: sent 4457811, received 107

I have done debug spanning-tree events on both switches and for almost 1h - I got nothing. However, switchA constantly reports (pretty much every minute) that there has been change in the topology (and I have not received even a single TCN/TCA..)

What do I miss here? :-)


Many thanks in advance!

Everyone's tags (4)
2 REPLIES
New Member

Hi Nikolay, the root switch

Hi Nikolay, the root switch is running RSTP or PVSTP+ ?

You can add the "spanning-tree portfast" command  to all non-uplink ports in Switch A to prevent receive BPDUs from ports that will never must participate in STP calculation.

New Member

Have we tried Try using  a

Have we tried Try using  a different port on switch B also when you join two different spanning tree domains your native vlan must match.

Im some cases I have it needs to be explicitly allowed over the trunks

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