[rbridge] it's time to summarize things

[Saikat Ray]

The simplest, if not the most rigorous, explanation is as follows. Replace
each legacy part of the network by a hub (i.e., first remove all RBridges
from the topology. Then choose a legacy bridge and replace this bridge and
all legacy bridges that have a path to it by a single hub, and let all
end-hosts served by those legacy bridges be connected to this hub. Repeat
this procedure until all legacy bridges are replaced. Then finally put the
RBridges back.). Now, one and only one RBridge forwards and receives
unencapsulated packets from each hub (through designated RBridge). Thus for
a loop formation, a packet must loop back to a designated RBridge, which is
not possible since RBridges use their own spanning tree for sending
(encapsulated) broadcast traffic.

Hope this helps.

> -----Original Message-----
> From: rbridge-bounces at postel.org [mailto:rbridge-bounces at postel.org] On
> Behalf Of Tim Shepard
> Sent: Thursday, December 15, 2005 6:42 PM
> To: Developing a hybrid router/bridge.
> Cc: Radia.Perlman at Sun.COM; 'Joe Touch'
> Subject: Re: [rbridge] it's time to summarize things
> 
> 
> > Let us suppose that an RBridge is connected to a legacy bridge's port
> > directly, and all RBridges block BPDUs. Then the RBridge looks like,
> from
> > the perspective of the legacy brige, either (i) a single end-host (with
> the
> > MAC address that of the RBridge) if the RBridge is not the designated
> > RBridge, or (ii) a shared media (like a hub; is the standard term
> > "segment"?) with potentially a lot of MAC addresses attached to it (in
> fact,
> [....]
> > So what exactly do you think is the problem with ARP?
> 
> 
> I'm not sure I follow all of the postings on this issue.
> 
> But after reading through this thread, I'm still left wondering what
> is supposed to make sure packets don't loop forever when there are a
> mix of bridges and rbridges plugged together (in some crazy accidental
> way, creating whatever kind of loop involving bridges and rbridges
> that causes the most trouble).
> 
> It seems to me that if rbridges block the spanning tree forming
> packets, but do forward (e.g.) ARP traffic, then there could be loops
> that are not broken by either the rbridge system or the usual bridge
> spanning-tree-forming algorithm.
> 
> Could someone explain succinctly (for someone who is just barely
> following this thread) how a system (of mixed bridges and rbridges,
> plugged together by some Byzantine net admin) will ensure that packets
> are not forwarded in a loop?
> 
> (And ARP is a good example, since it is supposed to go everywhere, but
> not loop.)
> 
> 
> Or will there be configuration rules that forbid some ways of plugging
> together bridges and rbridges?
> 
> 			-Tim Shepard
> 			 shep at alum.mit.edu
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