[nsp] PVST+ BPDU frame format

Tim Stevenson tstevens at cisco.com
Mon Mar 29 19:51:47 EST 2004

You are right, we actually call these "SSTP BPDUs". 

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 SSTP BPDU for the native VLAN of the trunk is sent untagged to the SSTP MAC address (0100.0ccc.cccd) on the native VLAN of the trunk
* Additional Cisco SSTP BPDUs, one for each of the other VLANs carried on the trunk, are sent 802.1Q tagged to the SSTP MAC address with the appropriate VLAN ID

The format of the SSTP 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 the source port (eg, if we send an SSTP 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.


At 09:00 AM 3/29/2004, cisco-nsp-request at puck.nether.net exclaimed:
>Message: 5
>Date: Mon, 29 Mar 2004 08:48:47 -0700
>From: "Clinton Work" <clinton at scripty.com>
>Subject: Re: [nsp] PVST+ BPDU frame format
>To: "Even Glemmestad" <even at supras.nu>, <cisco-nsp at puck.nether.net>
>Message-ID: <003901c415a5$52df4e90$d2d8b28e at corp.ads>
>Content-Type: text/plain;     charset="iso-8859-1"
>For compatibility reasons Cisco sends both regular BPDUs and PVSTP+ BPDUs on
>the native vlan (confirmed with a sniffer). The regular BPDUs are sent for
>switches which only have a single spanning tree instance for all VLANs. The
>BPDUs are sent on the native VLAN and every VLAN allowed on the trunk. The
>BPDUs use ethertype 0x010b and are sent to multicast MAC 01-00-0c-cc-cc-cd.
>----- Original Message ----- 
>From: "Even Glemmestad" <even at supras.nu>
>> Afaik, BPDUs sent from a Cisco over .1q trunks are not PVST BPDUs; when
>> Cisco uses a .1q trunk it uses normal STP on the link. I have created
>> connections between Cisco and 3rd-party switches using PVST+ on the
>> Cisco end, and STP on the 3rd-party end, running a .1q link in between.
>> The result, without any other configuration than setting the trunk mode
>> to dot1q, was that the BPDUs sent across the .1q trunks were normal STP
>> BPDUs for VLAN1, and interpreted correctly by the 3rd-party switches.

Tim Stevenson, tstevens at cisco.com
Routing & Switching CCIE #5561
Technical Marketing Engineer, Catalyst 6500
Cisco Systems, http://www.cisco.com
IP Phone: 408-526-6759
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