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New Member

loop-free topology with unmanaged switches

please see the attached figure.

there is a loop on the network. and unfortunately unmanagable access switches do not support stp/rstp.

i am trying to establish a loop-free architecture with what I have.

Altought it looks a dumb idea, what if I configure HSRP on core switches so make one switch active and oter one stand-by. looks like a loop-free network since one of the switches is not forwarding packets, there wont be a loop anymore?

would it work?

tx for helping :)

Cisco Employee

Re: loop-free topology with unmanaged switches

HSRP is meant to provide Layer 3 routing redundancy, so it will not help with mitigating layer 2 loops.

Looking at your diagram, STP on the core switches should cause one of the downstream ports connected to an unmanaged switch to block as the unmanaged switches should flood BPDUs out of every port. However, mixing switches that do not participate in STP with switches that do leaves a lot of vulnerability to layer 2 loops which can affect the entire network.

I would strongly recommend ensuring that all physical loops are eliminated from the unmanaged switches if this is feasible.



New Member

Re: loop-free topology with unmanaged switches

it looks there is no loop on this topology. even with unmanaged NO STP switches, Core switches with RSTP enabled prevents loop.

Re: loop-free topology with unmanaged switches


As already mentioned before, it depends on the "NO STP" switches whether there will be a loop (and immediate problems in the network) or not.

A) The "NO STP" switches forward BPDU used for spanning tree as they should - then no problem, the switches are from a STP perspective just an "active cable extender". Unless, of course, you create the loop between the "NO STP" switches.

B) The "NO STP" switches do NOT forward the BPDUs - then an immediate loop would be created.

Basically I would test it and go for it as long as everyone operating and maintaining the network is aware of the limitations.

Hope this helps! Please rate all posts.

Regards, Martin

New Member

Re: loop-free topology with unmanaged switches

thank you Martin.

Right, network would be down immediately if STP doesnt work.

I will test it and let you know the result.