Subject: [rbridge] Discovering endnodes directly connected to an RBridge

From: saikat at seas.upenn.edu (Saikat Ray)
Date: Thu, 6 Oct 2005 19:38:17 -0400 (EDT)

Hi All,

I am confused over a basic (probably a trivial) problem described below. I
would much appreciate if someone can answer.

Thanks in advance.

Saikat

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In the RBridge draft, page 4, it is written that "Once an RBridge learns
the location of a directly attached endnode, it informs the other RBridges
in its link state information." But how does an RBridge learn which
endnodes are directly connected to it using only promiscuous listening?
Suppose there is a LAN segment $Y$, such as a hub, connected to port $p$
of an RBridge $X$. Also assume that there is one another legacy bridge,
$Z$, connected to the LAN segment $Y$. The port of the legacy bridge $Z$
that connects it to the LAN segment $Y$ is in forwarding state, hence the
bridge $Z$ sends packets onto the LAN segment $Y$. Thus, the RBridge $X$
receives two types of packets on port $p$: (i) packets that are sent by
the endnodes that are directly connected to the LAN segment $Y$, and (ii)
packets that are forwarded by bridge $Z$ on LAN segment $Y$. Then how does
the RBridge $X$ distinguish between these two types of packets?

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From: touch at ISI.EDU (Joe Touch)
Date: Thu, 06 Oct 2005 16:47:29 -0700

> would much appreciate if someone can answer.

Looking for source addresses it has not seen yet, e.g, via broadcast ARP
requests.

> the RBridge $X$ distinguish between these two types of packets?

It doesn't care; both are effectively accessible off that egress of the
rbrige campus.

The term 'directly connected' above should probably be replaced with
'connected', since there's no way to distinguish. This will be addressed
in the new version of the doc which is split into separate problem
statement and arch docs.

Joe
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From: Radia.Perlman at sun.com (Radia Perlman)
Date: Thu, 06 Oct 2005 17:15:16 -0700

A legacy bridge would not remove the encapsulation header.

If there are two RBridges, only one is allowed to encapsulate from, and 
decapsulate onto, that link.

So therefore if an RBridge sees a packet without an encapsulation 
header, and that RBridge is the
Designated RBridge, then it knows the packet came from an endnode.

If the packet came from an endnode but was forwarded by a legacy bridge 
before reaching the first RBridge,
that's OK. It's the same as coming directly from the endnode. A "link" 
to an RBridge is a bunch of
Ethernet segments connected by legacy bridges.

Hopefully I understood your question. If not, please re-ask.

Radia

Saikat Ray wrote:

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>http://www.postel.org/mailman/listinfo/rbridge
>  
>

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From: saikat at seas.upenn.edu (Saikat Ray)
Date: Thu, 6 Oct 2005 20:51:16 -0400 (EDT)

(I am hoping that this post will go into the proper thread. I apologize
in advance if it starts a new thread.)

Thanks for your answers. You have properly understood the question.
However, I still have some doubts. Suppose a legacy bridge is connected to
(perhaps through a LAN segment) two RBridges. To use my example: the
legacy bridge $Z$ is connected to RBridges $X$ and $W$. Both the ports
that connect $Z$ to $X$ and $W$, respectively, are in forwarding state.
Now suppose a new endnode, $A$, joins a LAN segment connected only to the
legacy bridge $Z$, and the endnode sends out an ARP query. The legacy
bridge $Z$ forwards this query to both its ports. Thus both RBridge $W$
and $X$ receives this packet. Then isn't it the case that both $W$ and $X$
will think that the new endnode $A$ is "directly" connected to itself?

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From: touch at ISI.EDU (Joe Touch)
Date: Thu, 06 Oct 2005 20:23:18 -0700

> will think that the new endnode $A$ is "directly" connected to itself?

Yes. Which is why both will forward packets addressed to $A$ they
receive via other ports to the port they each have that attaches to the
legacy bridge $Z$, which ought to push that traffic - regardless of how
received - directly toward endnode $A$.

I don't see the problem yet.

Joe

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From: harald at alvestrand.no (Harald Tveit Alvestrand)
Date: Fri, 07 Oct 2005 11:02:36 +0200

--On torsdag, oktober 06, 2005 20:23:18 -0700 Joe Touch <touch at ISI.EDU> 
wrote:

>> However, I still have some doubts. Suppose a legacy bridge is connected
>> to (perhaps through a LAN segment) two RBridges. To use my example: the
>> and $X$ receives this packet. Then isn't it the case that both $W$ and
>> $X$ will think that the new endnode $A$ is "directly" connected to
>> itself?
>

Two questions come to my mind:

- Which node ends up in the routing tables of the other RBridges?
- Once the route is announced, will $X$ and $W$ believe the route or their 
directly acquired knowledge? And for how long?

(the first may be totally obvious to people who understand IS-IS....)

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From: touch at ISI.EDU (Joe Touch)
Date: Fri, 07 Oct 2005 09:16:13 -0700

> - Which node ends up in the routing tables of the other RBridges?

All external nodes potentially end up in the edge encapsulation tables.
Only the egress nodes within a particular rbridge campus will end up in
the edge and interior IS-IS routing tables, however, since only
encapsulated packets need be forwarded.

> directly acquired knowledge? And for how long?

I'm not sure whether $X$ and $W$ represent individual rbridge devices or
rbridge campuses. Up to this point in the thread, I had assumed
campuses, or devices on different campuses.

If they're on the same campus, what you have is what amounts to a loop
(remember - an rbridge campus acts like a single bridge to the outside
world). In that case, you have a misconfigured ethernet, since you're
connecting two bridges with two links.

Joe

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From: saikat at seas.upenn.edu (Saikat Ray)
Date: Fri, 7 Oct 2005 12:35:48 -0400

[Saikat] $X$ and $W$ are individual RBridge Devices. I think the question
was that in this example both $X$ and $W$ will advertise "direct
connectivity" to endnode $A$ to the rest of the RBridges. Then if some other
RBridge wants to send a packet to node $A$, which RBridge ($X$ or $W$) will
it send the packet to?

Could you also kindly answer the next question assuming that $X$ and $W$ are
individual RBridge Devices?

Joe

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From: touch at ISI.EDU (Joe Touch)
Date: Fri, 07 Oct 2005 09:56:31 -0700

> individual RBridge Devices?

I did:

	if they're in different campuses, there's no conflict

	if they're in the same campus, the external bridge $A$ is
	effectively connected twice to the same rbridge campus;
	this is out of spec.

I'm not sure how existing bridges handle this; I would expect that the
spanning tree ought to pick only one of those ports, i.e., there would
never be a case where any traffic would flow from $A$ to one of $X$ or
$Y$, at which point only one of ($X$,$Y$) would advertize any node
connected to $A$ anyway.

Joe

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From: Radia.Perlman at sun.com (Radia Perlman)
Date: Fri, 07 Oct 2005 10:16:57 -0700

I think I need a picture, but I think the question is what happens if 
there are two RBridges, W and X,
connected to the same link.

If that's the question, then the answer is that only one of them gets 
elected "Designated RBridge", say W. Then W
is the only one that learns endnode information (from listening to 
unencapsulated packets, whether they
be data packets or ARP/ND replies). W announces the endnodes on that 
link in W's link state information.

W is the only RBridge on that link that listens to unencapsulated 
packets, and the only one that decapsulates
packets when forwarding onto that link.

I'm confused about the "separate campuses" comment. If you connect 
RBridges, they'll all form one campus.
The only way to make it separate campuses is to partition them and only 
connect them through routers.

Radia

Joe Touch wrote:

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From: touch at ISI.EDU (Joe Touch)
Date: Fri, 07 Oct 2005 10:30:48 -0700

Radia Perlman wrote:
> elected "Designated RBridge", say W. 

I.e., the rbridge campus connects to A via only one link, as per
spanning tree rules (on the outside, a campus behaves like a single bridge).

> Then W

I am presuming that rbridges know which campus they belong to. Which
means we can deploy two sets of rbridges in separate campuses that are
either directly connected or connected via bridges.

We don't need a router to connect them. Or shouldn't, IMO. Is there some
reason that's a desirable property?

Joe

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From: saikat at seas.upenn.edu (Saikat Ray)
Date: Fri, 7 Oct 2005 13:54:12 -0400

I am not sure how Joe is defining "campus". The page 3 of the draft says "A
campus refers to a set of links connected by either RBridges or bridges. In
other words, the campus is terminated by traditional IP routers." The way I
interpret it: if a connected set of links is "campus A" and another set of
connected links (disjoint from the previous set) is "campus B", then the
only way from sending packets from an endnode in "campus A" to an endnode in
"campus B" is through an IP router. Am I misunderstanding something? If my
interpretation is correct, then I do not understand what is meant by "a
campus behaves like a single bridge"...

By the way, in my example, $A$ was an endnode, not an Rbridge or a Bridge.

Joe

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From: touch at ISI.EDU (Joe Touch)
Date: Fri, 07 Oct 2005 11:38:45 -0700

Saikat Ray wrote:
> other words, the campus is terminated by traditional IP routers." 

Yeah - that seems like a bug. It seems like an rbridge campus is a set
of links connecting either bridges or rbridges, terminated by rbridges.

i.e.,
	rbridges connected to internal bridges or rbridges
	internal bridges connected to internal bridges or rbridges
	all external links connected to rbridges only
		(declare these 'edge' rbridges')
	all nodes (hosts) connected via the external links

But there don't need to be IP routers per se.

Joe

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From: Radia.Perlman at sun.com (Radia Perlman)
Date: Fri, 07 Oct 2005 13:06:47 -0700

No. I think the wording is correct as is. There aren't explicitly 
"internal" and "external" RBridges. What you
are proposing would involve telling an RBridge which of its ports are 
"external" and which are "internal". At
any rate, it's not necessary. In a campus, there might happen to be a 
link which is a leaf, but if one were
to connect another RBridge to that link, you could grow the campus from 
that link.

So maybe we aren't disagreeing, since there certainly might be leaf 
links. But one could imagine a topology
in which there were no leaf links...every link has multiple RBridges and 
is used for transit, in addition to
possibly having endnodes on it. So trying to think of which links are 
termination links would just be confusing.

So RBridges do not terminate the campus. A router does, though, since 
the RBridges do not see anything
happening on the other side of the router.

Radia

Joe Touch wrote:

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From: gibanez at it.uc3m.es (Guillermo Ibáñez)
Date: Fri, 07 Oct 2005 22:38:13 +0200

Besides the reasons provided by Radia, there is the issue of a single 
and common IP subnet for the rbridge campus. If several campuses are 
connected by Rbridges instead of routers, all campuses will share the 
same IP subnet. This  does not seem a recommendable scenario, specially 
if different authorities own/manage the different campuses networks. I 
am not sure we use the term campus with the same meaning as Joe.
Guillermo

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From: touch at ISI.EDU (Joe Touch)
Date: Fri, 07 Oct 2005 16:31:04 -0700

> But there don't need to be IP routers per se.

FWIW, I did clarify this with Radia off-line. Rbridges campuses are
terminated at the rbridge, largely because of an IP router or host, but
it is at the rbridge that the termination exists.

As to the above,, here's a clarified version:

There are three kinds of L2 devices in an rbridge:

	hosts: anything that sources/sinks L2 packets
		e.g., RFC1122-style hosts, RFC1812-style routers
		(routers act like hosts to L2)

	bridges: transit conventional L2 packets and source/sink/transit
		spanning tree messages

	rbridges: encapsulate/decapsulate L2 packets with shim/L2 hdrs
		exchange routing via IS-IS,
		exchange egress tables via some protocol (TBD)

host-host and host-rbridge traffic
	may transit bridges
	is conventional L2

	this is known as EXTERNAL TRAFFIC

rbridge-rbridge traffic
	may transit bridges (since it looks like L2 on the outside)
	is shim/L2 encapsulated

	this is known as INTERNAL TRAFFIC

I hope this helps; it's a preview of the architecture doc ;-)

Joe

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From: Radia.Perlman at sun.com (Radia Perlman)
Date: Fri, 07 Oct 2005 16:43:55 -0700

RBridges are not intended to replace routers. If an organization is 
willing to do the configuration
necessary to partition their intranet into IP subnets, partitioned with 
routers, then they can/should
continue doing so. TRILL is really for more efficient/robust 
interconnection of links within an IP subnet.

Radia