Unanswered Question
May 29th, 2007
User Badges:

Hi Sir,

In a switched network running PVST+, I'm curious to know how does a switch differentiate BPDUs of one VLAN from another?

I understand that the Configuration BPDU message contains the following fields:

- Protocol ID

- Version

- Message Type

- Flags

- Root Bridge ID

- Root Path Cost

- Sender Bridge ID

- Port ID

- Message Age

- Maximum Age

- Hello Time

- Forward Delay

Please advise.

Thank you.


Lim TS

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Amit Singh Tue, 05/29/2007 - 09:51
User Badges:
  • Cisco Employee,


We actually send several different BPDUs on an 802.1Q trunk port:

* An 802.1d BPDU for VLAN 1 is sent untagged to the IEEE MAC address (0180.c200.0000) on the native VLAN of the trunk

* A Cisco STP BPDU for the native VLAN of the trunk is sent untagged to the STP MAC address (0100.0ccc.cccd) on the native VLAN of the trunk

* Additional Cisco STP BPDUs, one for each of the other VLANs carried on the trunk, are sent 802.1Q tagged to the STP MAC address with the appropriate VLAN ID

The format of the STP BPDU is 100% identical to the 802.1d BPDU after the SNAP header, except that we also add a "PVID" TLV field at the end of the frame, which identifies the VLAN ID of he source port (eg, if we send an STP BPDU on vLAN 10, the TLV contains vlan 10).

The TLV as the type (2 bytes), which is 0x0,the length (2 bytes) which is 0x2, and then 2 bytes of VLAN ID. There is also usually some padding in there.

HTH,Please rate if it does.

-amit singh

limtohsoon Wed, 06/11/2008 - 02:55
User Badges:

Hi Amit,

Thanks for the info. Can you please point me to the sources of the info for my further reading?

My customer experienced a brief network disruption today when they trunk a Catalyst 6509 to a Nortel Blade switch.

The Cisco port config is as follows:


int Gi12/13


switchport trunk encapsulation dot1q

switchport trunk allowed vlan 1-3,913,917

switchport mode trunk

switchport nonegotiate

spanning-tree guard root


It is not known to us the Nortel switchport config. However, we've been told they configured 802.1Q and run the same set of VLANs across the trunk. I suppose their Native VLAN (if I'm not mistaken, in Nortel term it is called PVID) is VLAN 1, same as Cisco.

I believe this is all that is needed to make CST on the Nortel switch to interoperate correctly with PVST+ on the Cisco switch.

The impact is some hosts on other VLANs lost network connectivity. I don't have the full picture here. However, stability is restored after VLAN 1 is disallowed from the trunk on Cisco side.

Are you able to relate anything here?

Thank you.


Lim TS


This Discussion