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What I've got:

Two 24 port switches in building A, with 2 fibre trunks from building A to building B - where there are two 48 port switches.

Diagram

The two 24 port switches support RSTP and STP. HP 1810-24G v2.

The two 48 port switches do not. HP 1810-48G v1.


What I want to do:

I want to add a trunk between 24 Port No 1 and 24 Port No 2 (using OM3-3) so that if OM3-1 or OM3-2 become disconnected or damaged, the packets will take the alternate path to the 48 port switches in building B.


Current issue:

When I add OM3-3, the switches lock up because I'm clearly creating a loop.


My Thoughts/Queries:

All switches are currently being managed on the same VLAN, which is 1080.

Will that solve my issue if I set the management VLAN on the two 24 port switches to say VLAN 2021, and leave the 48 ports on 1080 - then tag VLAN 2021 on the trunk between 48 Port No 1 and 48 Port No 2 so the switches can see each other?

I've had no experience with RSTP so I'm at a bit of a loss. The obvious solution is to replace the two 48 port switches with models which support RSTP and be done with it.

Is there any way to achieve this without replacing the 48 port switches?

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    Switches that don't support spanning tree sometimes drop the spanning tree packets but I think the 1810-48G should be OK in that respect. You did turn spanning tree on? – richardb Aug 28 '14 at 8:58
  • I will have a closer look at this and try and give you a detailed answer within the next 48h. – Max Aug 28 '14 at 9:57
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    Make sure you have RSTP turned on the ports connected to the 48p switches. The 1810s don't run STP, but they will pass BPDUs. So they will look like a piece of wire to the 24p switches. – Ron Trunk Aug 28 '14 at 12:26
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    Given your diagram, even if this will work, it will have a side effect that you may not anticipate. Since the two switches in building A support spanning-tree, either OM3-1 or OM3-2 will be in blocking, reducing your links between buildings to one. Replace the non-STP switches would be my recommendation. – YLearn Aug 29 '14 at 21:24
  • Thanks for the responses guys. Finally got back on to this today. No matter how I configure this (on the workbench!) I get mixed results, but mostly end up with a loop (as far as I can tell - a lot of flash lights and invariably one of the 4 switches becomes unreachable). – Sam Sep 4 '14 at 7:30
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RSTP is completely interoperable with STP. as per standards RSTP enabled port will go STP when connected to an STP enabled network.

I assume the older 1810 switches support STP and the 1810-v2 switches support RSTP, So in your setup the loop prevention should work fine.

Do config check on all switches to ensure they have the STP or RSTP enabled at switch level (global) as well as at port level.

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    "I assume the older 1810 switches support STP". No, they do not; they are right down in the bargain bin of managed switches. – richardb Aug 29 '14 at 20:35
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BPDU's should pass and one side should block, (most likely the side supporting RSTP). You can modify the port priorities to have one of the inter-building links blocking. If the 48 port switches do not support STP as long as the link between them is configured as a trunk BPDU's should pass. (access ports do not pass / process BPDU's).

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