Ethernet bridges and tagged BPDUs


The scenario: I would like to create a transparent L2 connection between two LANs via a third LAN, and run spanning tree between the endpoints without interfering with the configuration of the intermediate LAN switches. My idea is to VLAN tag the traffic between the LANs at the endpoints, including the STP traffic. In that way I hope that the intermediate switches will just forward the BPDUs used for STP, and not look into the contents of these frames.

The question: How will an Ethernet switch react to incoming BPDUs that are VLAN tagged? Or put another way: Will the swicthes in the intermediate LAN

1) Change their STP configuration according to the BPDU 2) Discard the BPDU 3) Forward the BPDU with the VLAN tag preserved 4) or...?

I cannot seem to find anything in the IEEE standards about this.

If this doesn't work, are there alternatives?

BR, Mike Absalon

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Yes. Do not use STP. (As long as this is the only connection it is no use anyway).

The KISS principle rules. Just build your networks in the most simple and stupid way possible. If anything breaks, let it break. Your average downtime will be significantly less than the outages introduced by the ill implemented features promising redundancy in todays equipment.

Reply to
Manfred Kwiatkowski

It may be possible to do what you are trying to do depending on which switches you are using. Prior to multi-instance spanning tree in IEEE 802.1Q, spanning tree PDUs (or BPDUs) were never tagged. Since they are identified by bridges because of their special MAC address, a bridge would have been expected to: (a) never generate tagged BPDUs, and (b) discard any received BPDUs that were tagged.

With newer switches that implement multi-instance spanning tree, you are allowed to configure "spanning tree domains" and it's possible you may be able to tunnel the BPDUs from one spanning tree domain across another. (I don't think the spec says anything about this, though.)

I think you'd have to very carefully look into what your equipment does. There isn't an easy yes/no answer to this.


Reply to
Anoop Ghanwani

the implementation in various switches is specific to the manufacturer, varies by software release and config - and is not usually documented at the level needed to check what you want to do.

you probably need to bench test this to find out what will happen.

there is an alternative - several manufacturers support multiple 802.1q labels in a single packet - this setup is intended to let a carrier build a layer 2 shared network for multiple customers.

Since one of the reasons for using label stacking (or Q in Q, or various other names) is to insulate the carrier from customer Spanning Trees - it should do what you want.

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