STP Loop with Third Party Switches I manage a network mostly comprised of older Avaya ERS 3500 series switches. Recently another department has bought a package that comes with it's own server rack, 2 switches, and servers. The vendor tells me that all they need is 2 trunk ports that allow VLAN 84 to flow through. One port dedicated to SW1 and another to SW2 Every time I have plugged their switches into the server access switch, it immediately causes an STP loop and takes my network down. The problem is that another company is in charge of managing the Cisco switches that they have. I have provided a rough drawing of the setup below.

[Core(VSP)] ------------ [Server Access (ERS 3500)]==========[Unman SW 1(Cisco)] -[Unman SW 2(Cisco)]

My question is, what can I do on my server access switch to prevent the STP loop from on my end?

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    Dec 15, 2019 at 4:59

2 Answers 2


1) Enable STP on your switches

2) Access cisco SWs and check:

  • if STP is enabled and if it is compatible.

  • Check if portfast is enabled and disable it on trunk interfaces

  • Disable spanning tree bpdufilter on trunk interfaces

3) Try to bundle the links between Avaya and Cisco SW

  • Aggregating likely won't work when the Avaya has two Ciscos connected.
    – Zac67
    Jul 18, 2019 at 13:50
  • It depends. They might be stacked.And you should aggree that topology is little bit confusing... Jul 18, 2019 at 13:54

You need to at least take a look at the Cisco configs - if they use RPVST+ and the Avayas can't, the protocol must be changed to something both sides support - MSTP or RSTP. RPVST does not interoperate with RSTP/MSTP when using VLAN trunking. Without any config on the Cisco side ("Unman"?) it won't ever work.

If you can't get either RPVST or R/MSTP on all switches you can't run those redundant links. If you do, check out Viktor's tips for portfast and bpdufilter.

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