[rbridge] Ingress Rbridge address and BCN

[Caitlin Bestler]

rbridge-bounces at postel.org wrote:
> Interesting. In theory, it is possible to send a packet to an
> unknown destination MAC by sending through the ingress
> RBridge tree rooted at the RBridge that decided to send the
> BCN (or rooted at the Designated RBridge on the link on which
> some legacy bridge decided to send the BCN).
> 
> But if there are going to lots of BCN's, then it would cut
> down on traffic to know the original ingress RBridge, since
> the BCN's would never needed to be sent everywhere---just
> direct to the ingress RBridge.
> 
> Just to make sure I do understand though, the intent is that
> the BCN does need to get to the original source of the
> packet, and the ingress RBridge of the original packet will
> only forward the BCN to the source, right?
> 
> Radia
> 
> 
> 

My analasys is that an 802.1au-capable RBridge could send
a BCN in three different ways:

1) RBridge encapsulated to the original source c/o its 
   source RBridge, which it already knew. The egress RBRidge
   would typically have this information.
2) non-RBridge encapsulated is sent to the outer source,
   which is the *prior* rbridge and not necessarily the
   source rbridge, and we specify that 802.1au-capable
   RBridges act as proxies for BCNs received that are
   not RBridge encapsulated.
3) RBridge encapsulated to the original source, but with
   an unknown destination RBridge.

I believe that #2 is required anyway, to deal with 802.1au
bridges that are not RBridges.

It is also worht pointing out that unless there are *four*
RBridges in the "rbridge path" that the immediate prior
rbridge to any core rbridge *will* be the source rbridge.

I believe these three options provide adequate solutions
to this problem without requiring the addition of the
source rbridge to the frame. But it does suggest that
the header format should be designed so that it stacks
efficiently with 802.1a headers.

Standardizing the 802.1au proxy role is also important
because otherwise you would have Rate Limited Frame
wrappers around RBridge encapsulation. I believe it 
is far better to ensure that the BCN is forwarded
to the original source so that RBridge encapsulates
the Rate Limited Frame.