STP Query

[jatinder.sharma]

i need some help to resolve STP related query.  we have Two Switch in which everything is same like MAC address ( Varitual MAC ), Route ID, interface cost, ( SW 1 interface f0/1 connected to Sw 2 int f0/2 & Sw 1 inter f0/2 connected to Sw2 int f0/1), then which port will be block by STP, pls help.

[Rolf Fischer]

Duplicate post: https://supportforums.cisco.com/discussion/12734251/stp-query

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Broadly speaking, there are 3 steps to STP convergence:

1. Elect a single root bridge
2. Elect the (single) root port on every switch
3. Determine the port roles and states within the segments between the switches

You're asking about step 3, but in your scenario step 1 cannot take place adequately, because both switches will consider themself as root bridges. However, STP should be able to handle even this, because every switch will behave as if it received back it's own BPDU due to a direct physical loop. So you will result in two (logical) standalone-switches and Backup/Blocking ports.

If the MAC address are virtual, you should be able to change them because MAC addresses of devices in a LAN must be unambiguous. From a STP perspective you could change the priority on one switch, but this would not solve the non-STP problems.

HTH
Rolf

[milan.kulik]

Hi Rolf,

I believe only one port is blocked in a case of the switch received back it's own BPDU due to a direct physical loop (and no advanced STP features like BPDUguard or Loopguard are enabled)?

According to a nice description here

http://mrncciew.com/2013/07/07/stp-root-port-selection/

it should be the port with lower intefrace number in fact?

So in the hypothetical excercise described in the original post where two switches would use the same MAC address for STP and being connected: SW 1 interface f0/1 connected to Sw 2 int f0/2 & Sw 1 inter f0/2 connected to Sw2 int f0/1, the result would be:

SW 1 interface f0/1 blocked

Sw2 int f0/1 blocked

and no data passing between the switches.

While in a case of SW 1 interface f0/1 connected to Sw 2 int f0/1 & Sw 1 inter f0/2 connected to Sw2 int f0/2, the result would be:

int f0/2 blocked on both switches

and data passing through interfaces f0/1 between the switches.

Correct?

Best regards,

Milan

[Rolf Fischer · Accepted solution]

Hi Milan,

thanks for your contribution! And yes, I believe your're absolutely correct ...

As we know, the port ID is the very last tie breaker, so in this case the Fa0/2 BPDUs are inferior and they should result in ALT/BLK (and of course stop sending BPDUs then).

I guess with Fa0/1-Fa0/1 and Fa0/2-Fa0/2 connections we would see non-deterministic results with one port in each connection in ALT/BLK.

Best regards,
Rolf

[jatinder.sharma]

Thanks Mr. Milan Kulik for your support & again thanks to all.