IEEE 802.1ad-2005 “Provider Bridges” is a completed amendment to 802.1Q.
(not to be confused with the in process 802.1ah “Provider Backbone Bridges” amendment)
Provider Bridges outer S-Tags and are transparent to some bridging multicast addresses.
“Provider Edge Bridges” talk C-Tags (formerly called Q-Tags) on customer ports and S-Tags on provider ports.
Should it be possible to configure Rbridges as Provider or Provider Edge Bridges and/or incrementally replaced such bridges with RBridges?