[rbridge] RBridge: a case of study

[Eastlake III Donald-LDE008]

See below at ### 

-----Original Message-----
From: James Carlson [mailto:james.d.carlson at sun.com] 
Sent: Monday, November 05, 2007 10:16 AM
To: Eastlake III Donald-LDE008
Cc: Developing a hybrid router/bridge.
Subject: RE: [rbridge] RBridge: a case of study

Eastlake III Donald-LDE008 writes:
> From: James Carlson [mailto:james.d.carlson at sun.com] 
[...]
> Eric Gray writes:
[...]
> > 2) all frames forwarded from a (physical) port on one RBridge 
> >    to a (physical) port on another RBridge MUST have the same
> >    VID, or 
[...]
> @@@ Well, I am Donald and I meant something like (2) but a bit
narrower.
> I would have said "all TRILL frames" because "all frames" is way too
> inclusive. What about control frames like 802.1AB? Furthermore, in the
> case of point-to-point links between two Rbridges (i.e., no bridges or
> end station connections in between), as far as I can see there is no
> particular utility in considering the TRILL frames to have any
external
> VLAN coloring and certainly no need for them to be outer tagged with a
> VLAN ID or priority.

I mostly agree with that.  In fact, I think that's a desirable end
state for TRILL, when all other bridges are obsolete.  Having no VLAN
tag on the outside makes a lot of sense there.

(I think omitting the priority makes a bit less sense to me, though.
Wouldn't you want the priority value set on an 802.1q header with VLAN
ID 0 in this case?)

### The way I look at it, the VLAN coloring and priority of a frame are
meta data associated with the frame. The priority is used in queuing
decisions related to processing and transmitting the frame. Just what
Q-tag gets put on the transmitted frame depends on configuration.

### If a link is point-to-point between Rbridges, there is no need for
an outer VLAN ID. If it is a data frame, the actual VLAN ID of the data
is in the inner Q-tag and so easily available to the next Rbridge for
its decision processes. Similarly, I don't see any reason for priority
in the outer Q-tag. It's not as if a piece of wire is going to look at
the priority and re-order frames. If it is a data frame, the actual
priority of the data is in the inner Q-tag and so is easily available to
the next Rbridge.

### TRILL core IS-IS frames, of course, are different. They don't have
any inner Q-tag because the don't have any inherent VLAN coloring and
they only go one hop anyway. An Rbridge might want to associate
"priority 7" or whatever as meta data with the core IS-IS frame while it
is internal to the Rbridge and that could effect transmission queuing
but there is no particular reason for there to be a priority tag on it
over a point-to-point link. The next Rbridge will see that it is a core
IS-IS message, will probably treat it a bit differently for processing,
and give it to its core IS-IS instance.

### Thanks,
### Donald

I think the question is how to get there when there are networks that
are half-TRILL and half-otherwise.

...

-- 
James Carlson, Solaris Networking              <james.d.carlson at sun.com>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677