1. Light ENNI – aka Split LAG …
Region C 3 b b
Region A c
Link
Aggregation
Basic Forwarding Rules :
a frame from the network is loaded into a locally-attached LAG link :
if the whole LAG on that node fails, it blocks its “Region-facing” ports
on egress from a LAG, a frame may have to pass over an IST (==) to ensure that it enters the destination region via the correct port :
only one port can be open to any destination, otherwise frame looping and duplication start, and MAC learning breaks;
MAC learning is also shared and synchronised over the IST
ISTs must be proof against single point of failure :-“split brain” issue :
modulo Norm’s “suicide pact” suggestion;
they can also be built as LAGs in their own right if desired

2. Light ENNI – aka Split LAG …
Region C 3 b b
LAG
Region A c
Link
Aggregation
The easiest way to access this structure is “triangular” Split LAG :
exactly one frame is sent in, and exactly one frame comes out
But this looks very “heavy” for a light ENNI 
The alternative is to make {a, b, c …} full participants in their region’s control regimes :
which if this is xSTP looks “interesting”; next slide

3. Light ENNI – aka Split LAG …
Region C 3 b b
Region A c
Link
Aggregation
The alternative is to make {a, b, c …} full participants in their region’s CP :
xSTP is the challenging case
There can only be a single unblocked point of attachment of the virtual bridge (3a, 3b, 3c ) to the region’s Spanning Tree on any VLAN :
the structure above duplicates frames on flooding ,
and “thrashes” source learning for frames travelling 
The Spanning Tree root cannot be placed “on” a virtual bridge element :
the Region may have > 1 E-NNI

4. Light ENNI – aka Split LAG …PN
Region C 3 b b
Region A c
Link
Aggregation
In a Link State environment, the solution is straightforward :
all bridges in Region C can obtain a common view of which of {a, b, c} is on the shortest path to a “PN”, and act accordingly
When running xSTP, the ENNI probably has to do this “by the back door” :
the xSTP on {a, b, c} has to be extended to allow “private” sharing of Root Path Priority Vector between a  b, a  c, and b  c :
with the winner being the owner of the unblocked port;
unless there is a better idea … ?

5. “One we made earlier”

6. SPBM for resilient PB E-NNI
NW 1
PB NW 2
S-Tag = x
S-Tag = y
I-SID = z
Operators 1 & 2 bind each PB S-tag to a common SPBM I-SID
Each operator is responsible for advertising that I-SID once only into the SPBM domain :
but each operator has complete authority as to where that is
So long as a viable path remains available in the SPBM domain, connectivity will automatically be maintained by SPBM.
I-SiDs are provisioned in SPBM :
a signalled interface might be desirable
MVRP ?