[rbridge] it's time to summarize things
touch at ISI.EDU
Fri Dec 16 13:29:21 PST 2005
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Saikat Ray wrote:
> 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.
Here's a variant that maps to only existing technologies:
- remove the rbridges (as Saikat said)
this leaves some number of disconnected
bridged ethernet segments
- replace the entirety of each bridged ethernet segment
with a single hub (again, as Saikat said)
I'm not positive this is valid; it will
be if you replace the segment with a bridge, though.
I'm worried about doubly-connected hosts, etc.
- replace the rbridge campus with a bridge
FORCE that bridge to be the root of the spanning tree
*IF* there is at most one rbridge campus in a LAN *AND* that campus can
end up knowing it is root of all the edge spanning trees, there can't be
The key are the IF and AND clauses.
The IF is effectively forced by the HELLO messages; all rbridge devices
will coalesce into a single campus. This means you can't use multiple
campuses to handle scalability in a LAN, however.
The AND is assumed; if there is any other device that tries the same
trick and makes the same assumption (whether another rbridge, or an
rbridge-like system that doesn't use rbridge messages), the assumption
will be invalid.
> Hope this helps.
>>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,
>>>the perspective of the legacy brige, either (i) a single end-host (with
>>>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
>>>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
>>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
>>Or will there be configuration rules that forbid some ways of plugging
>>together bridges and rbridges?
>> -Tim Shepard
>> shep at alum.mit.edu
>>rbridge mailing list
>>rbridge at postel.org
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