Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

RSTP(Rapid Spanning Tree) is configured on a bridge. All ports of this bridge are "trunk" ports and are part of vlan “x”. Will the BPDUs sent out of this bridge be tagged?

share|improve this question
I think you're missing the point of this site. It's not a Wiki to list questions and answers as trivia, it's a tool to ask questions that represent real, actual problems that need solving. – Bigbio2002 Jun 12 '12 at 22:07
@Bigbio2002 - incorrect. You are totally allowed to share your knowledge here in q/a by answering your own questions. There's even a special button to click when you're asking that makes this even easier. Not only is it permitted, it's encouraged by the Stack Exchange network. – Mark Henderson Jun 13 '12 at 3:57
True... but this guy was getting a lot of downvotes, so I figured that was the reason why. Not questions for the sake of trivia, but questions for a purpose. Which begs the question, why all the downvotes? – Bigbio2002 Jun 13 '12 at 4:14

No. RSTP BPDUs are never tagged irrespective of the port type(access/trunk). Because RSTP BPDUs on being received by a bridge are sent to the CPU for processing if the bridge understands RSTP or are discarded. So, they need not be forwarded.

share|improve this answer
I think 802.1q tags are present on BPDUs in Cisco's PVST+? – Shane Madden Jun 13 '12 at 2:55
On Juniper Networks switches, i have verified that it does not send out tagged BPDUs. This seems correct because when you have different ports on a bridge part of different VLANs, and you enable RSTP on the bridge, then all the ports of the bridge are treated as if they were all in same VLAN for all RSTP convergence events. I mean that if two ports in 2 separate VLANs are connecting to the same bridge on the other side, then RSTP will block one port and forward data only on the other port. – gsinha Jun 13 '12 at 3:10
PVST+ is RSTP running separately in each individual VLANs. So, separate spanning trees are formed for each VLAN in case of Cisco's PVST+ or Juniper's VSTP. – gsinha Jun 13 '12 at 3:13
Sounds right! It might be worth adding a note to the answer that those variants on RSTP behave differently. And I'm not sure how MST behaves - I assume there's no tags, but it might be worth confirming! – Shane Madden Jun 13 '12 at 3:19
MST maintains a single topology for the VLAN's and ports assigned to it. PVST(+) is configured on a per-VLAN basis. The win with the former is a lot fewer BPDU's being propagated, which improves scalability considerably. Depending on the implementation both can be implemented as rapid (802.1w) or standard (802.1d) spanning tree. – rnxrx Jun 13 '12 at 3:58

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.