RSTP and Trunk Links

[Adam_Martin]

Hi All,

New to Cambium gear. I have been reading and can see that trunk ports operate slightly different as opposed to in the Cisco sense. All VLANs are tagged and although you can set a native VLAN it only receives untagged frames it will not transmit them.

How does this work then when it comes to RSTP and BPDUs being sent between switches? and what VLAN are they sent on?

Thanks

[TamN]

Hi Adam,

Your understanding the behavior of cnmatrix trunk port is correct. However, the switch sends protocol packets such as RSTP, LLDP etc. untagged.

Thanks

[Adam_Martin]

Hi Tam,

Thanks for confirming so RSTP,LLDP and any other management traffic what VLAN will these be part of?

Would it be on VLAN 1 or for example would it be VLAN 99 (example if we set VLAN 99 to our Management VLAN). Would this essentially mean that whatever the management VLAN is set as that it would go over the trunk link untagged and in actual fact the trunk would send untagged frames?

[TamN]

Hi Adam,

The management traffic is tagged on a trunk port. I mentioned RSTP, LLDP as they are vlan independent.

If you require to have management traffic egressing untagged, port must be configured for hybrid mode. Please note that a port in hybrid mode is not automatically added to every VLANs present in the switch, and user must configure the hybrid port in selected VLANs. Please refer to this link for explanations of different port mode supported on cnmatrix :

cnMatrix Switchport modes explained

Summary: This article describes the cnMatrix switchport modes and gives examples of how and when to use them. Each port on a cnMatrix switch can be configured as access, trunk, or hybrid port. Here's a description of these port types: Access Mode A port that can be assigned to a single VLAN. It accepts and transmits only untagged frames. Access port is typically connected to desktop PC or devices incapable of handling tagged frames. Trunk Mode A port that is auto associated to ALL VLANs. It a…

Thanks

[Adam_Martin]

Thanks Tam

So for RSTP traffic for example BPDUs over a trunk if these are sent untagged then essentially this is the native vlan of the link? Is this vlan 1?

I am just wanting to clarify my understanding as we are looking at deploying Cambium across all our sites so I want to know exactly how it works

Thanks

[TamN]

Hi Adam, RSTP BPDU is sent untagged and will be received in the port’s native/pvid vlan. Please note that with RSTP, unlike PVRST and MSTP, all VLANs are part of the same STP domain.