RSTP - no blocked Port

[whistleblower]

Hi all,

I have a constellation in which 4 switches are connected in a ring - but there is no loop although on a Meraki switch port, RSTP is deactivated on the interface! No idea but maybe the different native vlan on the trunk plays a role? 🤔

thanks in advance for any help!

[DarrenOC]

Hi @whistleblower 

As per below, if the port has RSTP disabled then it won’t participate in STP. It’s this config that’s stopping the loop

Option	Description
Enabled	RSTP must be enabled globally (see "Enable RSTP Globally") for any ports to be able to participate in Spanning Tree processes. When RSTP is enabled globally, RSTP will be enabled at the port level by default. A disabled port can be re-enabled by selecting Enabled. While RSTP is enabled on a switch port, that port is able to participate in Spanning Tree processes. It is recommended that RSTP be enabled on all ports.
Disabled	RSTP may be disabled at the port level. Disabling RSTP on a port removes the port from any STP processing including any STP guard configuration. Disabling RSTP on a port is not recommended unless the client device connected to the port is incompatible with STP. If RSTP is disabled globally, all ports will have RSTP disabled and cannot have it enabled.

[whistleblower]

Hi, @DarrenOC thank you for your reply on this topic! But what I don't understand is that RSTP is disabled on the port and the connected device speaks STP ... so I don't understand why it works without problems / loop at all!

[whistleblower]

someone any ideas on that?

[Bruce]

Since only VLANs 2 and 3 are allowed on the trunks on the left-hand side of the diagram, a loop has definitely been formed for these two VLANs based on the diagram. Are these VLANs in use? Are these any other ports on the four switches that RSTP has put into blocking state? What model switches are they, has someone enabled ‘broadcast storm controls’ to make the network workable (although obviously not correct)? Are there any messages in the Event Logs?

[whistleblower]

I`ve just looked at the configuration on the bottom switch again and I`ve to say that I made a mistake in the sketch! the port (towards where RSTP is disabled) is an access port untagged in vlan 1

[Bruce · Accepted solution]

In that case you don’t have a loop 😀

On the left-hand side of the diagram you only have VLANs 2 and 3, in the top right segment you have all VLANs on the wire, and in the bottom right you only have VLAN1 (since it’s an Access Port in VLAN1 it only carries VLAN1). 

RSTP (if enabled) runs on a port regardless of whether it’s an access port or a trunk port. The BPDUs it sends are always un-tagged, but it’s not part of a VLAN, although it does impact all VLANs. In your scenario, if RSTP was enabled on the port you have indicated it is disabled on, it would likely go into blocking state on one of the uplinks of the bottom switch; this is even though there isn’t an actual Layer 2 loop, there is a loop of ‘un-tagged’ traffic.

I wouldn’t call it good practice, but it is working as would be expected in this scenario.

[whistleblower]

@Bruce thank you man for that detailed explanation! I appreciate that help on this topic very much! 👍

Just for my better understanding - when RSTP is disabled on a switchport, does this automatically mean that there is some sort of BPDU-Filter active?

[Bruce]

When RSTP is disabled on a port, the port takes no part in spanning-tree, so it always passes traffic. It won’t send BPDUs, and it won’t process incoming BPDUs (so essentially they get dropped). The recommendation is generally not to disable RSTP as if you do get a loop it will take down your network.