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Explorer

stp problem

I am seeing two different switches claiming themselves to be spaning tree root for the same vlan (using same mac address). Can anybody please explain how this is possible?

Switch1#sh spanning-tree vlan 192

Spanning tree instance for vlan 192

VL192

  Spanning tree enabled protocol rapid-pvst

  Root ID    Priority    32960

             Address     001c.7319.9d54

             This bridge is the root

  Bridge ID  Priority    32960  (priority 32768 sys-id-ext 192)

             Address     001c.7319.9d54

             Hello Time  2.000 sec  Max Age 20 sec  Forward Delay 15 sec

Switch2#sh spanning-tree vlan 192

Spanning tree instance for vlan 192

VL192

  Spanning tree enabled protocol rapid-pvst

  Root ID    Priority    32960

             Address     001c.7319.9d54

             This bridge is the root

  Bridge ID  Priority    32960  (priority 32768 sys-id-ext 192)

             Address     001c.7319.9d54

             Hello Time  2.000 sec  Max Age 20 sec  Forward Delay 15 sec

7 REPLIES 7
Beginner

stp problem

This may be a silly question, but are the switches connected? Are they allowing the vlan to traverse the connection? Are you using VTP? What's the output of 'sh vtp status' of both switches?

Engager

stp problem

If all the switches use the default priority (0x8000 / 32768), the MAC-address is needed as the tie-breaker.

MAC-Adresses of course should be unique, otherwise with default configruration there is no tie-breaker and the bridges remain claiming themselves as rootbridges (no superior BPDU received).

Are that real switches or is it a simulation (packettracer, gns3)?

What about the other VLANs; do they also have the same MAC-Adresses?

Regards

Rolf

Advocate

Re: stp problem

Hi,

if you issue "show version" on the switches, do they both show the same Base ethernet MAC Address?

BR,

Milan

Explorer

Re: stp problem

They show different mac addresses.

001c.7319.9d54 is shown on switch1..switch2 shows different mac

Advocate

Re: stp problem

Hi,

how are the switches connected each to the other?

Aren't they in stack, e.g.?

BR,

Milan

Explorer

Re: stp problem

Switches are connected to each other, but they are not in stack.

I think it's because of mlag. thanks for your help!

Advocate

Re: stp problem

Oh, you are running MLAG there?

What kind of switches are those?

7600s?

BR,

Milan

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