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I am having a problem that I do not quite understand with my network setup.

I have four switches on the network: Switch A is a managed network switch which is connected to the PC router -- it is set to its factory defaults with no advanced configuration. Switch B is an unmanaged network switch which is connected to the phone router. Switch C & Switch D are both configured with VLANs 1 and 10. Ports in each VLAN on both Switch C and Switch D are connected to ports on switches A and B as follows:

Switch C port 24 (VLAN 10) -> Switch A port 48

Switch C port 23 (VLAN 1) -> Switch B port 1

Switch D port 24 (VLAN 10) -> Switch A port 47

Switch D port 23 (VLAN 1) -> Switch B port 2

VLAN 1 Network: 10.10.20.0/24

VLAN 10 Network: 10.20.30.0/24

I am only using the VLAN functionality to logically separate ports on Switches C & D into two different broadcast domains. The Switches are configured to untag the packets on egress, so the upstream switches (A & B) should have no concept of the VLANs.

The problem I am having is that Switch C and Switch D are unable to connect to Switch A simultaneously, whereas both are able to connect to Switch B without issue.

Can anyone provide insight on why this could be?

  • It sounds like you didn't set the STP priority for the VLANs. You want to make sure that Switch A is the root switch for the VLANs. You need to set the STP priority low for both the VLANs on Switch A. – Ron Maupin Jun 14 '16 at 2:05
  • I'm also confused about VLAN 1. It doesn't seem to have any way to get to Switch A, or the router. Do you have a separate connection from Switch A to Switch B? You should probably trunk between Switch A and Switches C and D. – Ron Maupin Jun 14 '16 at 2:18

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