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VLAN Tag vs PVST+ , IEEE BPDU

Hello all

 

cisco website says,

"""""This list shows how PVST+ interoperates with IEEE 802.1Q or IEEE 802.1D, if the Native VLAN on an IEEE 802.1Q trunk is VLAN 1:

  • VLAN 1 STP BPDUs are sent to the IEEE STP MAC address (0180.c200.0000), untagged.

  • VLAN 1 STP BPDUs are also sent to the PVST+ MAC address, untagged.

  • Non-VLAN 1 STP BPDUs are sent to the PVST+ MAC address (also called the Shared Spanning Tree Protocol [SSTP] MAC address, 0100.0ccc.cccd), tagged with a corresponding IEEE 802.1Q VLAN tag.

If the Native VLAN on an IEEE 802.1Q trunk is not VLAN 1:

  • VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.

  • VLAN 1 STP BPDUs are also sent to the IEEE STP MAC address on the Native VLAN of the IEEE 802.1Q trunk, untagged.

  • Non-VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.

    Note: Native VLAN STP BPDUs are sent untagged.""""

 

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24063-pvid-inconsistency-24063.html#topic1

https://www.cisco.com/en/US/docs/ios/lanswitch/configuration/guide/lsw_rtng_vlan_ovw_ps6350_TSD_Products_Configuration_Guide_Chapter.html#wp1006501

 

But in my lab test, I found the following results

can anyone explain this, please

SW: 3550

STP mode: RSTP

Native VLAN: 3

Allowed VLAN: 1-3

Trunk type: Dot1Q

Pc OS: Linux 

"Vlan dot1q tag native" command applied.

 

As per the website:

If the Native VLAN on an IEEE 802.1Q trunk is not VLAN 1:

  • VLAN 1 STP BPDUs are  sent to the IEEE STP MAC address on the Native VLAN of the IEEE 802.1Q trunk, untagged.

My lab test:

VLAN 1 STP BPDUs are  sent to the IEEE STP MAC address on the Native VLAN of the IEEE 802.1Q trunk, untagged (because I enabled native VLAN tagging) <--- AgreedNV31-min.jpg

 

As per the website:

If the Native VLAN on an IEEE 802.1Q trunk is not VLAN 1:

  • Non-VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.

 

My Lab test:

VLAN 2 sstp bpdu sent with Tag 2

VLAN 3 sstp bpdu alslo sent with Tag 3 (because I enabled native VLAN tagging) <--- AgreedNV32-min.jpg

NV33-min.jpg

 

Next

As per the website:

If the Native VLAN on an IEEE 802.1Q trunk is not VLAN 1:

  • VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.

 

My Lab Test:

VLAN 1 stp bpdu sent to the PVST+ MAC address without any tag!-----I have a confusion here.

 

 NV34-min.jpg

 

Also, I changed native VLAN back to 1

now

  • IEEE VLAN 1 BPDU  is not tagged (but I still enabled "vlan dot1q tag native" command) 
  • Other VLAN SSTP BPDUs were tagged correctly.
  • And VLAN 1 SSTP BPDU was not tagged! (in the previous test  Native vlan 3 tagged but it didn't happen for VLAN 1)NV11-min.jpg

    NV12-min.jpg

    NV13-min.jpg

     

what should I conclude?!!

I couldn't get that result in real-time, please help me to understand.

 

+I studied control packets like VTP, CDP  always uses VLAN 1, but I couldn't find any tag on these packets while native vlan 3.


Thanks 

Siva

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