Cisco – Native VLAN Mismatch between two access ports on cisco switches

Hope I understand what you are asking here (your writing is long and my memory is short :))

The HP untagged VLAN on a tagged interface functions similarly as the native VLAN you configure on Cisco switch trunk interface (actually tag/untag makes more sense than access/trunk technically speaking), the VLAN 8 in your case.

On your router interface facing HP switch, it is running as access/untagged port. So all ingress traffic on that router interface is VLAN 69; meantime router will remove VLAN 69 tag of the egree traffic to switch and when traffic arrives on switch, it will be VLAN 8. However if you have traffic marked with other VLAN, like VLAN 10 and send the traffic to router as the interface facing router is tagged/trunk port, router will not be able to understand it as the edge interface is expecting something without VLAN tag.

There is no native VLAN mismatch just like you connect a laptop to a trunk interface with certain native VLAN configured and switch does not complain about mismatch, right?

Hope this helps.

The non-Cisco switch will be running MST if it is using Rapid STP. You need to make sure your Cisco switches are also running MST, not the default PVST+ or Rapid PVST+. You need to run the same STP version on all the connected switches for STP to work correctly.

Once you fix all that, make sure that the native VLANs on each end of a link are the same. Cisco switches will use CDP to verify this, but your non-Cisco switch probably doesn't have CDP, so it may ignorant of the mismatch.

EDIT per the comment:

Different versions of STP use different BPDUs to establish the root switch and root ports. Cisco PVST+ (including Rapid PVST+) uses a separate STP instance for each VLAN, while MST uses one instance of STP for each user-defined group of VLANs, and, unless you take pains to do this, the two STP versions probably won't match in this regard. When mixing the two, it is normally best to use MST, and correctly match the VLANs to the STP groups. This works well, but it is certainly more work than using the Cisco STP version everywhere.

It is possible to interoperate the two versions, but it can be difficult to get it right so that the root and root ports are correctly identified on each switch in the layer-2 domain. Having inconsistent root switches and root ports can cause STP loops resulting in broadcast storms which can bring down the entire layer-2 domain.

It is much easier to use a single STP version across all your switches, and, unfortunately for added complexity, that would probably be be MST when mixing Cisco and non-Cisco switches.