What does a switch do if it receives inferior BPDUs from itself?

[droeun141]

Say switch A and C are connected via switch B (which has spanning-tree disabled) and a loop is created on B.  What does switch A do when it receives inferior BPDU's from itself?

[Gurpreet Kochar]

Please confirm if this is how the topology looks as per your example

SWITCH-A---------------SWITCH-B------------------SWITCH-C

Or, SwitchA and Switch C are directly connected to each other as well

SwitchA -------------Switch-C

     \                       /

       \                    /

         \                /

           \             /

            Switch-B(spanning tree disabled)

[droeun141]

Yeah only except A & C don't connect directly to each other, only through switch B which has STP disabled

[Gurpreet Kochar]

If A and C are not connected to each other, the loop never completes

The BPDU sent out by the switch A is received on int 1 of switchB and goes out int 2 on switchB to switchC

The Switch C sends its BPDU out through int 2 on Switch B and it reaches switch A via int 1.

A switch does not send the traffic back the interface it receives on, therefore once the BPDU leaves the the switch it does not see it back unless there is an alternate path.

[droeun141]

Thanks Gurpreet - I undstand that though... but my question was what if a loop was created on switch B? (ports 9 & 10 on switch B inadvertently connected), and B has spanning tree turned off.  BPDU from switch A reaches B on 1/1 and gets looped on ports 9 & 10 on switch B before going back out 1/1 and hitting switch A again.  What happens then?

[Gurpreet Kochar]

A switch can not send superior and inferior BPDU at the same time

The superiority or the inferiority of the BPDU depends upon the combination of two factors

1. Priority of the switch

2. Mac addres of the switch

You may want to refer to the following link

http://www.cisco.com/image/gif/paws/10556/spanning_tree1.swf

[Gurpreet Kochar]

Thank you for attaching the diagram

As per the above diagram, i believe the switch should loop back error message and put the interface into error disabled state.

[Peter Paluch · Accepted solution]

Dear friends,

Please allow me to join the discussion.

First, the words "superior" and "inferior" are relative: anytime we speak about superior and inferior BPDUs, we have to say what is that particular BPDU superior/inferior to. There is no superior or inferior BPDU per se, rather, of each two BPDUs sent by two different ports (on the same switch or on different switches), one is superior to another (and vice versa, the second is inferior to the first). The design of STP makes sure that each two BPDUs can be compared and decided which of them is superior/inferior to the other (in other words, the BPDUs create a total ordering).

Second, per the diagram, a BPDU sent by the Switch A may loop on the Switch B and get back to the Switch A. If the BPDU is received by another port than the one that sent the BPDU, normal election of a Designated/Backup port would ensue. However, if the BPDU is received by the same port that originally sent it, the STP will declared the port as Broken (Looped) and will put it into Blocking state. It will not be err-disabled.

If you decide to try this out experimentally, you may find that the port on Switch A eventually gets err-disabled; however, that will not be caused by the STP but by an independent mechanism: each port on Catalyst switches emits so-called LOOP frames. We have been in extensive discussions with Giuseppe Larosa about its purpose and each one of us still seems to be somehow biased about it Nevertheless, if a port receives a LOOP frame it has sent itself, it will be err-disabled immediately.

So in this topology, the STP would merely block the port, however, the LOOP mechanism would sooner or later shut it down completely.

Best regards,

Peter

[Gurpreet Kochar]

Awsome!!!!

Thanks for the detailed explanation

[droeun141]

Thanks Peter! great explanation.