[rbridge] VLAN mapping anonmolies...does anyone care?

[Donald Eastlake]

Hi Anil,

On Mon, Dec 14, 2009 at 9:28 AM, <Anil_R at 3com.com> wrote:

> I'd say it would be best to define it in such a way that, once a switch has
> been given a VLAN mapping from one port to another, it either rejects a
> different mapping or overrides the old one, depending on how the MIB is
> defined. Which is to say, make it "invertible" always.


I guess I don't understand the first part of what you say above. Making it
always be invertible sounds safe but it seems to me there might be cases
where you can't do that.

Say that you have two LANs with 3,000 VLANs in common that need to be mapped
but each having 1,000 local VLANs that have no corresponding VLAN in the
other LAN. It seems to me that you can't make an invertible mapping to do
this. The best you can do is just to map most of the local VLANs to oblivion
at the border and arrange your networks so the probability is very low that
the shortest path between any two RBridges in of these LANs goes through the
other LAN.

So, my current thinking it that the mapping SHOULD be invertible.


> In practice, this sort of connectivity when desired by customers is done in
> a way that doesn't require VLAN mapping: you'd define "overlapping VLANs"
> which allow, in this example, VLAN A as well as VLAN B to reach the ports
> of VLAN C, but do not allow VLAN A and VLAN B to talk to each other (if
> that were the requirement.)
>

That sounds like VLAN groups, which are touched on in Section 4.8.3 of the
-14 version of the base protocol draft. I don't see it as having much to do
with VLAN mapping (also known as VLAN ID translation).


> In another scenario, you may have 3 different sites East, West and South,
> where West wants its VLAN A to be translated to VLAN C in the East and
> South wants its VLAN B to be translated to the same VLAN C in the East
> (vice versa for the opposite direction). In such a case you would have 3
> switches (connecting East, West and South), each of which have invertible
> mappings. This is a much more common application of VLAN mapping: site
> administrators define their own VLANs, then their edge switches translate
> when necessary to connect into VLANs in a different site.
>

Sure, handling N different regions isn't a problem.


> Regards,
> Anil
>

Thanks,
Donald

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> rbridge-bounces at postel.org wrote on 12/12/2009 06:12:31 PM:
>
> > Radia Perlman <Radia.Perlman at Sun.COM>
> > Sent by: rbridge-bounces at postel.org
> >
> > 12/12/2009 06:12 PM
> >
> > To
> >
> > rbridge at postel.org
> >
> > cc
> >
> > Subject
> >
> > [rbridge] VLAN mapping anonmolies...does anyone care?
> >
> > A couple of things to bring to people's attention in fewer words than
> > reading the VLAN mapping draft. :-)
> >
> > a) if mappings are not invertible, as in, both VLAN A and VLAN B in East
> > map to VLAN C in West,
> > then it's not clear what to map VLAN C to, when forwarding from West to
> > East.
> >
> > I'd say either make that a configuration error and don't let someone
> > configure two VLANs that map
> > to the same thing, or say that when mapping from (West, VLAN C), if both
> > A and B from East map to (West, C),
> > then map to the highest VLAN number, or the first one listed in the
> > mapping configuration table. Other
> > suggestions?
> >
> > b) if mappings are not invertible, same example as a). and the path goes
> > from East to West and then East
> > again, VLAN A in East might wind up getting mapped to VLAN B in East.
> >
> > I don't care -- I think noninvertible mappings should be illegal.
> >
> > c) another form of noninvertible mapping is (East, VLAN A : West, null),
> > meaning that VLAN A in East
> > should be dropped instead of being forwarded to cloud West. The
> > surprising result of this, if the path
> > goes from East to West and back to East, is that VLAN A might become
> > partitioned as a result.
> >
> > I don't care, especially since I can't think of any good, simple
> > solution to this.
> >
> > Radia
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>
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