Unanswered Question
May 1st, 2007

hello how mstp,rstp technically transported over isl and dot1q? tx

I have this problem too.
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Francois Tallet Wed, 05/02/2007 - 08:53

MSTP BPDUs are sent untagged on 1Q trunk (I'm trying to avoid the term native vlan, which is not exactly correct). On ISL links, they are sent on vlan 1 afaik.

Rapid-PVST, which is the PVST version of RSTP, is sending one BPDU per vlan. The BPDU for vlan 1 is sent untagged on 1Q trunks (with the regular IEEE STP mac address as a destination), while the other vlans (!=1) have their BPDUs sent tagged, to a cisco proprietary mac address.

On ISL trunks, the BPDUs for all the vlans are sent in their respective vlan, with the IEEE mac address as a destination.

Hope this helps,


Francois Tallet Thu, 05/03/2007 - 09:55

Untagged frames are not really sent on a "native vlan", they just don't have an 802.1Q header. If you want, vlans are just an additional layer on the top of the mac layer. Not a great analogy, but for instance, UDP works on the top of IP and you don't call an IP packet a "native UDP frame";-)

Bridges that are not 1q capable just ignore the 1q tag, while 1q bridges classify the untagged frames they receive in a given vlan. STP untagged control frames are received by the control protocol even before the switch gets to classify untagged frames into a particular vlan. That's why I would not say they belong to "the native vlan" (btw, the concept of a unique native vlan is Cisco specific, in theory several vlans could be sent untagged on a particular port).




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