STP related question

Question

Hi this is a portion of my current set up..

All vlan ports are untagged. Everything works and communicates fine until a connection is made between the 2 switches it takes down the network and the pc is no longer able to ping the gateway. The network functions when STP is enabled on ALL 3 device, but if it is disabled on my VOIP then the network goes down again. Wouldn't STP being enabled on just my gateway be enough to stop this outage(loop?)? The switches are different VLAN names but all ports are untagged. Any insight would be great.

Answer 1

Disabling STP and creating a loop will crash the network with a broadcast storm. STP was designed to prevent layer-2 loops by enabling a single loop-free path to the root bridge.

STP is communicated on a link-by-link basis, and disabling it on even one link can cause a broadcast storm.

In any case, loops and lines are not preferred. If you want to disable STP, you need to create a loop-free design using trees. Even then, an accident of connecting two switches at a patch panel or wall plate can create a broadcast storm, so STP is a failsafe in that case.

Answer 2

xSTP relies on passing BPDUs between switch ports. Managed switches don't usually forward those frames (as it should be), so a managed switch without STP in between two others with STP active breaks functionality.

Simple, "dumb" switches often forward BPDUs like other frames (in violation of 802.1D), so they may enable STP switches to maintain their spanning tree.

In a nutshell, you should always activate STP. Basic configuration includes selecting a proper root bridge by priority and ensuring overall operation, especially when mixing several STP flavors (classic STP, RSTP, MSTP, or R/PVST/+).

The only argument against using xSTP are potential delays on link-up, disturbing e.g. DHCP. On HPE/Aruba switches, you might want to activate the admin-edge option for end-node facing switch ports (the equivalent of portfast on Cisco gear).