What happens when STP is disabled on one side

[daudparvez]

Hi All,

I have a scenario that a Layer 2 switch is connected to an OFS long distance system. The OFS long distance system also behaves like a Layer 2 switch. STP is disabled on OFS system. Different branch sites aggregate on the Switch which are then taken to a hub site using OFS. Now if i need to connect two FE Trunks between the Switch and the OFS what will be the behaviour of STP keeping in mind that STP is disabled on one side?

Beste Regards,

Daud Parvez

[Reza Sharifi]

Hi Daud,

If this OFS device supports link aggregation you can put both links between the switch and OFS in a Portchannel.

This way, you don't need STP.

HTH

[daudparvez]

Hi Raza,

Thanks for reply,but due to certain reasons i cannot budle the two links into a port channel. Sorry for not mentioning this beforfe.

Best Regards

[Vivek Ganapathi]

Hi Daud,

This would end up in a loop without having STP enabled on your OFS system. Which model of Layer 2 switch you have? Determine the switch model & if the feature set of "Flex-links" are supported for the platform, you can think of using it.

One of the redundant link would be in blocked state. So, it's fine to have your OFS system with STP disabled.

Regards

Vivek

[narainarun]

Hi Vivek,

With STP disabled on OFS system, shall it not treat BPDU's as normal multicast and forward BPDU's back to switch where STP enabled and the upstream switch will block one of the ports? I read somewhere when STP is disabled the switch still forwards BPDU as it treats that as normal multicast.

HTH

Arun

[Vivek Ganapathi]

You are right Arun. I take back those words.

[smitesh kharecha]

Hi Daud,

If STP is enabled on your Switch, it will block one of the link ( even though OFS has STP disabled) as soon as it detected the STP loop.

HTH,

Smitesh

[daudparvez]

Thanks all for your kind support.

But i am still confused that how would the STP enabled switch detect a loop if the OFS is not forwarding any of it's own BPDUs. Even if the OFS System replicates the BPDUs generated by the STP Enabled switch back to it, would'nt the STP Enabled switch consider itself as Root and keep both it's ports as Designated/Forwarding? whereas the OFS system with STP disabled would also keep both its ports Forwarding since it is the STP which actually blocks ports potentially causing loops which is disabled in  this scenario?

[smitesh kharecha · Accepted solution]

Hi Daud,

I tried replicating the scenario in the lab ( apart from OFS, what i did was connected a switch and disabled the spanning tree on it, basically making it behave like a transparent bridge) and saw that Switch who is the root block the port with the highest interface.

HTH,

Smitesh

[narainarun · Accepted solution]

Hi Daud,

When OFS (with STP disabled) floods multicast packets (STP BPDU's) back to the switch which runs STP, then treat OFS has a kind of hub (only as far as BPDU/multicast frames) concerned. Thus a switch which has a self looped on its two ports will block one of the port. As per STP algorithm, the port with highest port ID is blocked.

Remember in STP, by default all ports in Root switch is designated except in one condition where root bridge has a self loop port. As you recall in RSTP, there is a port state called Backup which says the BPDU's are received from the same switch but still inferior. So the port with highest Port ID is blocked as confirmed by Smitesh.

Hope this helps

Arun

pls rate if it helps