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Wednesday, March 23, 2011

How BPDU is transmitted with Native VLAN for PVST and MSTP

Qsolved Question: If native VLAN is VLAN 1, How BPDU is transmitted when using Cisco proprietary protocol PVST+ or IEEE protocol MSTP? If native VLAN is changed to VLAN 999 , how is it then? If native VLAN is changed to VLAN 999 and vlan 1 is not allowed on the trunk, how is it then? And how other traffic will separately be affected with native vlan 1 and native vlan 999?

Qsolved Answer:

Regarding the MSTP, it is easy: the MSTP sends all its BPDUs untagged, regardless of the native VLAN on the trunk. Information about individual instances is contained as so-called Mrecords inside the single MSTP BPDU, and the entire BPDU is sent untagged.The PVST+ is more complicated. I invite you to read the following two documents:

Click here for the document 1. And the second document can be found here.

In short, if the native VLAN is VLAN1 then:
  • VLAN1 standard STP BPDU is sent untagged
  • VLAN1 PVST+ BPDU is sent untagged
  • Other VLAN's PVST+ BPDUs are sent tagged with their appropriate VLAN

If the native VLAN is different from VLAN1 then:

  • VLAN1 standard STP BPDU is sent untagged
  • VLAN1 PVST+ BPDU is sent tagged with VLAN1
  • Other VLAN's PVST+ BPDUs are sent tagged accordingly (the one for the native VLAN will be untagged).

In short, the standard STP BPDU is always derived from VLAN1 and is always sent untagged. The PVST+ BPDUs are derived from their appropriate VLANs and are tagged according to the native VLAN on the trunk.

Citation - This blog post does not reflect original content from the author. Rather it summarizes content that are relevant to the topic from different sources in the web. The sources might include any online discussion boards, forums, websites and others.

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