weird PVST issue

[CSCO11508096]

Hi everyone,

I am getting loop in my network inspite of running pvst.

PVST is running on two switches that are connected to each other through etherchannel po1.

on one switch root port is po1 as it priority is high. similar configuration is done on other switch too but another root port is elected on second switch.

bothe the switches have same bridge priority.

Why two ports are elected as a root, and how can i rectify the issue.

please let me know if any other imformation is required.

SW01#sh spanning-tree vlan 80

VLAN0080

  Spanning tree enabled protocol ieee

  Root ID    Priority    32768

             Address     0019.aa2d.a6f0

             Cost        7

             Port        56 (Port-channel1)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32848  (priority 32768 sys-id-ext 80)

             Address     001e.f6d7.3300

             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               Desg FWD 4         128.1    P2p

Gi0/3               Desg FWD 4         128.3    P2p

Gi0/4               Desg FWD 4         128.4    P2p

Gi0/5               Desg FWD 4         128.5    P2p

Gi0/24              Desg FWD 4         128.24   P2p

Gi0/25              Desg FWD 4         128.25   P2p

Po1                 Root FWD 3         128.56   P2p

SW02#sh spanning-tree vlan 80

VLAN0080

  Spanning tree enabled protocol ieee

  Root ID    Priority    32768

             Address     0019.aa2d.a6f0

             Cost        4

             Port        5 (GigabitEthernet0/5)

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32848  (priority 32768 sys-id-ext 80)

             Address     001f.26b2.9880

             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               Desg FWD 4         128.1    P2p

Gi0/2               Desg FWD 4         128.2    P2p

Gi0/3               Desg FWD 4         128.3    P2p

Gi0/4               Desg FWD 4         128.4    P2p

Gi0/5               Root FWD 4         128.5    P2p

Gi0/8               Desg FWD 4         128.8    P2p

Gi0/26              Desg FWD 4         128.26   P2p

Interface           Role Sts Cost      Prio.Nbr Type

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

Po1                 Desg FWD 3         128.56   P2p

Thanks

Shashi

[Rolf Fischer]

Hi Shashi,

I guess the topology looks like this:

Root-Bridge  (0019.aa2d.a6f0) -----------------Gi0/5-|-Sw2-|-Po1--------------Po1-|-Sw1 |

There is only one root bridge in the bridging-domain (per VLAN), but every non-root switch elects the one port with the best path to the root-brigde, this port is called the root-port.

Gi0/5 is the root-port on Sw2 because it has the lowest cost to the root bridge (4).

Po1 is the root-port on Sw1 because it has the lowest cost ot the root bridge (3 + 4 = 7).

HTH

Rolf

[CSCO11508096]

HI everyone,

one thing here to notice is no switch become root.

[CSCO11508096]

Hi fischer,

Thanks for ur reply...

actuly 0019.aa2d.a6f0 is a IPS device connected to switch.

Thanks

Shashi

[Rolf Fischer · Accepted solution]

actuly 0019.aa2d.a6f0 is a IPS device connected to switch.

That's interesting. The output of the show-commands you've posted show that it acts as a spanning-tree enabled bridge, or maybe it works in a transparent layer-2 mode and just forwards BPDUs from the other side.

If you  want to make one of your Switches Sw1/Sw2 to be the root brigde, you can use the macro

spanning-tree vlan root primary

but I'd suggest to do that in a maintenance window.

HTH

Rolf

[CSCO11508096]

Hi,

switch2 connected to ISP switch that is acting as a root.

but this will happen only when he is also running vlan 80 and pvst. Am I right???

Also please let me know if any problem in making my Sw1 or Sw2 as a root.

Thanks

Shashi Mishra

[Rolf Fischer]

Now I'm confused: Is it a IPS or a ISP switch?

However, from Sw2's perspective it is belongs to VLAN 80, if the Sw2's bridge priority is the lowest in VLAN 80, it will take over as the new root bridge.

This will work regardless of the STP mode on the other side.

I'd also like to note that, as long as you don't have physical loops for redundancy, the placement of the root bridge is more a cosmetic question as it doesn't change any forwarding path.

Best regards

Rolf